GLASS-GRINDING AND OPTICIANS' WORK
§ 52. As no instructions of any practical value in this art have, so
far as I know, appeared in any book in English, though a great deal of
valuable information has been given in the English Mechanic and
elsewhere, I shall deal with the matter sufficiently fully for all
practical purposes. On the other hand, I do not propose to treat of
all the methods which have been proposed, but only those requisite for
the production of the results claimed. The student is requested to
read through the chapter before commencing any particular operation.
§ 53. The simplest way will be to describe the process of manufacture
of some standard optical appliance, from which a general idea of the
nature of the operations will be obtained. After this preliminary
account special methods may be considered in detail. I will begin
with an account of the construction of an achromatic object glass for
a telescope, not because a student in a physical laboratory will often
require to make one, but because it illustrates the usual processes
very well; and requires to be well and accurately made.
A knowledge of the ordinary principles of optics on the part of the
reader is assumed, for there are plenty of books on the theory of
lenses, and, in any case, it is my intention to treat of the art
rather than of the science of the subject. By far the best short
statement of the principles involved which I have seen is Lord
Rayleigh's article on Optics in the Encyclopaedia Britannica, and this
is amply sufficient.
The first question that crops up is, of course, the subject of the
choice of glass. It is obvious that the glass must be uniform in
refractive index throughout, and that it must be free from air bubbles
or bits of opaque matter. [Footnote: The complete testing of glass
for uniformity of refractive index can only be arrived at by grinding
and polishing a sufficient portion of the surfaces to enable an
examination to be made of every part. In the case of a small disc it
is sufficient to polish two or three facets on the edge, and to
examine the glass in a field of uniform illumination through the
windows thus formed. Very slight irregularities will cause a "mirage"
easily recognised.]
The simplest procedure is to obtain glass of the desired quality from
Messrs. Chance of Birmingham, according to the following abbreviated
list of names and refractive indices, which may be relied upon:-
Density. Refractive Index.
C D F G
Hard crown
2.85 1.5146 1.5172 1.5232 1.5280
Soft crown
2.55 1.5119 1.5146 1.5210 1.5263
Light flint
3.21 1.5700 1.5740 1.5839 1.5922
Dense flint
3.66 1 6175 1.6224 1.6348 1.6453
Extra dense flint
3.85 1.6450 1.6504 1.6643 1.6761
Double extra dense flint
4.45 1.7036 1.7103 1.7273 ...
The above glasses may be had in sheets from 0.25 to 1 inch thick, and
6 to 12 inches square, at a cost of, say, 7s. 6d. per pound.
Discs can also be obtained of any reasonable size. Discs 2 inches in
diameter cost about £1 per dozen, discs 3 inches in diameter about
10s. each. The price of discs increases enormously with the size. A
16-inch disc will cost about £100.
For special purposes, where the desired quality of glass does not
appear on the list, an application may be made to the Jena Factory of
Herr Schott. In order to give a definite example, I may mention that
for ordinary telescopic objectives good results may be obtained by
combining the hard crown and dense flint of Chance's list, using the
crown to form a double convex, and the flint to form a double concave
lens. The convex lens is placed in the more outward position in the
telescope, i.e. the light passes first through it.
The conditions to be fulfilled are:
(1) The glass must be achromatic;
(2) it must have a small spherical aberration for rays converging to
the principal focus.
It is impossible to discuss these matters without going into a
complete optical discussion. The radii of curvature of the surfaces,
beginning with the first, i.e. the external face of the convex lens,
are in the ratio of 1, 2, and 3; an allowance of 15 inches focal
length per inch of aperture is reasonable (see Optics in Ency.
Brit.), and the focal length is the same as the greatest radius of
curvature. Thus, for an object glass 2 inches in diameter, the first
surface of the convex lens would have a radius of curvature of 10
inches, the surface common to the convex and concave lens would have a
radius of curvature of 20 inches, and the last surface a radius of
curvature of 30 inches. This would also be about the focal length of
the finished lens. The surfaces in contact have, of course, a common
curvature, and need not be cemented together unless a slight loss of
light is inadmissible.
I will assume that a lens of about 2 inches diameter is to be made by
hand, i.e. without the help of a special grinding or polishing
machine; this can be accomplished perfectly well, so long as the
diameter of the glass is not above about 6 inches, after which the
labour is rather too severe. The two glass discs having been obtained
from the makers, it will be found that they are slightly larger in
diameter than the quoted size, something having been left for the
waste of working.
It is difficult to deal with the processes of lens manufacture without
entering at every stage into rather tedious details, and, what is
worse, without interrupting the main account for the purpose of
describing subsidiary instruments or processes. In order that the
reader may have some guide in threading the maze, it is necessary that
he should commence with a clear idea of the broad principles of
construction which are to be carried out. For this purpose it seems
desirable to begin by roughly indicating the various steps which are
to be taken.
(1) The glass is to be made circular in form and of a given diameter.
(2) Called Rough Grinding. The surfaces of the glass are to be made
roughly convex, plane, or concave, as may be required; the glass is to
be equally thick all round the edge. In this process the glass is
abraded by the use of sand or emery rubbed over it by properly shaped
pieces of iron or lead called "tools."
(3) The glass is ground with emery to the correct spherical figure as
given by a spherometer.
(4) Called Fine Grinding. The state of the surface is gradually
improved by grinding with finer and finer grades of emery.
(5) The glass is polished by rouge.
(6) The glass is "figured." This means that it is gradually altered in
form by a polishing tool till it gives the best results as found by
trial.
In processes 2 to 5 counterpart tool surfaces are required--as a rule
two convex and two concave surfaces for each lens surface. These
subsidiary surfaces are worked (i.e. ground) on discs of cast iron
faced with glass, or on slate discs; and discs thus prepared are
called "tools."
Taking these processes in the order named, the mode of manufacture is
shortly as follows:-
(1) The disc of glass, obtained in a roughly circular form, is mounted
on an ordinary lathe, being conveniently cemented by Regnault's mastic
to a small face plate. The lathe is rotated slowly, and the glass is
gradually turned down to a circular figure by means (1) of a tool with
a diamond point; or (2) an ordinary hand-file moistened with
kerosene, as described in § 42; or (3) a mass of brass or iron served
with a mixture of emery--or sand--and water fed on to the disc, so
that the disc is gradually ground circular.
The operation of making a circular disc of given diameter does not
differ in any important particular from the similar operation in the
case of brass or iron, and is in fact merely a matter of turning at a
slow speed.
(2 and 3) Roughing or bringing the surfaces of the glass roughly to
the proper convex or concave shape. This is accomplished by
grinding, generally with sand in large works, or with emery in the
laboratory, where the time saved is of more importance than the value
of the emery.
Discs of iron or brass are cast and turned so as to have a diameter
slightly less than that of the glass to be ground, and are, say, half
an inch thick. These discs are turned convex or concave on one face
according as they are to be employed in the production of concave or
convex glass surfaces. The proper degree of convexity or concavity
may be approximated to by turning with ordinary turning tools, using a
circular arc cut from zinc or glass (as will be described) as a
"template" or pattern. This also is a mere matter of turning.
The first approximation to the desired convex or concave surface of
the glass is attained (in the case of small lenses, say up to three
inches diameter) by rotating the glass on the lathe as described above
(for the purpose of giving it a circular edge) and holding the tool
against the rotating glass, a plentiful supply of coarse emery and
water, or sand and water, being supplied between the glass and metal
surfaces. The tool is held by hand against the surface of the
revolving glass, and is constantly moved about, both round its own
axis of figure and to and fro across the glass surface. In this way
the glass gradually gets convex or concave.
The curvature is tested from time to time by a spherometer, and the
tool is increased or decreased in curvature by turning it on a lathe
so as to cause it to grind the glass more at the edges or in the
middle according to the indications of the spherometer.
This instrument, by the way--so important for lens makers--consists
essentially of a kind of three-legged stool, with an additional leg
placed at the centre of the circle circumscribing the other three.
This central leg is in reality a fine screw with a very large head
graduated on the edge, so that it is easy to compute the fractions of
a turn given to the screw. The instrument is first placed on a flat
plate, and the central screw turned till its end just touches the
plate, a state of affairs which is very sharply discernible by the
slight rocking which it enables the instrument to undergo when pushed
by the hand. See the sketch.
On a convex or concave surface the screw has to be screwed in or out,
and from the amount of screwing necessary to bring all four points
into equal contact, the curvature may be ascertained.
Let a be the distance between the equidistant feet, and d the distance
through which the screw is protruded or retracted from its zero
position on a flat surface. Then the radius of curvature rho is given
by the formula 2rho = a2/3d +d.
Fig. 43.
The process of roughing is not always carried out exactly as
described, and will be referred to again.
(4) The glass being approximately of the proper radius of curvature on
one side, it is reversed on the chuck and the same process gone
through on the other side. After this the glass is usually dismounted
from the lathe and mounted by cement on a pedestal, which is merely a
wooden stand with a heavy foot, so that the glass may be held
conveniently for the workman. Sometimes a pedestal about four feet
high is fixed in the floor of the room, so that the workman engaged in
grinding the lens may walk round and round it to secure uniformity.
For ordinary purposes, however, a short pedestal may be placed on a
table and rotated from time to time by hand, the operator sitting down
to his work.
Rough iron or brass tools do not succeed for fine grinding--i.e.
grinding with fine emery, because particles of emery become embedded
in the metal so tightly that they cannot be got out by any ordinary
cleaning. If we have been using emery passing say a sieve with 60
threads to the inch, and then go on to some passing say 100 threads to
the inch, a few of the coarser particles will adhere to the "tool",
and go on cutting and scratching all the time grinding by means of the
finer emery is in progress.
To get over this it is usual to use a rather different kind of
grinding tool. A very good kind is made by cementing small squares of
glass (say up to half an inch on the side), on to a disc of slate
slightly smaller than the lens surface to be formed (Fig. 51). The
glass-slate tool is then "roughed" just like the lens surface, but, of
course, if the lens has been roughed "convex" the tool must be roughed
"concave".
The "roughed" tool is then used to gradually improve the fineness of
grinding of the glass. For this purpose grinding by hand is resorted
to, the tool and lens being supplied continually with finer and finer
emery. Fig. 52 gives an idea of the way in which the tool is moved
across the glass surface. Very little pressure is required. The tool
is carried in small circular sweeps round and round the lens, so that
the centre of the tool describes a many-looped curve on the lens
surface. The tool must be allowed to rotate about its own axis; and
the lens and pedestal must also be rotated from time to time.
Every few minutes the circular strokes are interrupted, and simple,
straight, transverse strokes taken. In no case (except to correct a,
defect, as will be explained) should the tool overhang the lens
surface by more than about one quarter the diameter of the latter.
After grinding say for an hour with one size of emery fed in by means
of a clean stick say every five minutes, the emery is washed off, and
everything carefully cleaned. The process is then repeated with finer
emery, and so on.
The different grades of emery are prepared by taking advantage of the
fact that the smaller the particles the longer do they remain
suspended in water. Some emery mud from a "roughing" operation is
stirred up with plenty of water and left a few seconds to settle, the
liquor is then decanted to a second jug and left say for double the
time, say ten seconds; it is decanted again, and so on till four or
five grades of emery have been accumulated, each jug containing finer
emery than its predecessor in the process.
It is not much use using emery which takes more than half an hour to
settle in an ordinary bedroom jug. What remains in the liquid to be
decanted is mostly glass mud and not emery at all. The process of
fine grinding is continually checked by the spherometer, and the art
consists in knowing how to move the grinding tool so as to make the
lens surface more or less curved. In general it may be said that if
the tool is moved in small sweeps, and not allowed to overhang much,
the Centre of the lens will be more abraded, while if bold free
strokes are taken with much overhanging, the edges of the lens will be
more ground away.
By the exercise of patience and perseverance any one will succeed in
gradually fine grinding the lens surface and keeping it to the
spherometer, but the skill comes in doing this rapidly by varying the
shape of the strokes before any appreciable alteration of curvature
has come about.
Polishing.
The most simple way of polishing is to coat the grinding tool with
paper, as will be described, and then to brush some rouge into the
paper. The polisher is moved over the work in much the same way as
the fine grinding tool, until the glass is polished. Many operators
prefer to use a tool made by squeezing a disc of slate, armed with
squares of warm pitch, against the lens surface (finely ground), and
then covering these squares with rouge and water instead of emery and
water as in the fine grinding process.
The final process is called "figuring." It will in general be
unnecessary with a small lens. With large lenses or mirrors the final
touches have to be given after the optical behaviour of the lens or
mirror has been tested with the telescope itself, and this process is
called "figuring." A book might easily be written on the optical
indications of various imperfections in a mirror or lens. Suffice it
to say here that a sufficiently skilled person will be able to decide
from an observation of the behaviour of a telescope whether a lens
will be improved by altering the curvature of one or all of the
surfaces.
A very small alteration will make a large difference in the optical
properties, so that in general "figuring" is done merely by using the
rouge polishing tool as an abrading tool, and causing it to alter the
curves in the manner already suggested for grinding. There are other
methods based on knocking squares out of the pitch-polisher so that
some parts of the glass may be more abraded than others.
The "figuring" and polishing may be done by hand just like the
grinding. There are machines, however, which can be made to execute
the proper motions, and a polisher is set in such a machine, and the
mechanical work done is by no means inconsiderable. In fact for
surfaces above six inches in diameter few people are strong enough to
work a polisher by hand owing to the intense adhesion between it and
the exactly fitting glass surface.
Such is a general outline of the processes required to produce a lens
or mirror. These processes will now be dealt with in much greater
detail, and a certain amount of repetition of the above will
unfortunately be necessary: the reader is asked to pardon this. It
will also be advisable for the reader to begin by reading the whole
account before he commences any particular operation. The reason for
this is that it has been desirable to keep to the main account as far
as possible without inserting special instructions for subsidiary
operations, however important they may be; consequently it may not
always be quite clear how the steps described are to be performed. It
will be found, however, that all necessary information is really
given, though perhaps not always exactly in the place the reader might
at first expect.
§ 54. All the discs that I have seen, come from the makers already
roughly ground on the edges to a circular figure--but occasionally the
figure is very rough indeed--and in some cases, especially if small
lenses have to be made, it is convenient to begin by cutting the glass
discs out of glass sheet, which also may be purchased of suit-able
glass. To do this, the simplest way is to begin by cutting squares
and then cutting off the corners with the diamond, the approximate
circular figure being obtained by grinding the edges on an ordinary
grindstone.
If the pieces are larger, time and material may be saved by using a
diamond compass, i.e. an ordinary drawing compass armed with a
diamond to cut circles on the glass, and breaking the superfluous
glass away by means of a pair of spectacle-maker's shanks (Fig. 44),
or what does equally well, a pair of pliers with soft iron jaws. With
these instruments glass can be chipped gradually up to any line,
whether diamond-cut or not, the jaws of the pincers being worked
against the edge of the glass, so as to gradually crush it away.
Fig. 44.
Assuming that the glass has been bought or made roughly circular, it
must be finished on the lathe. For this purpose it is necessary to
chuck it on an iron or hardwood chuck, as shown in Fig. 46. For a
lens below say an inch in diameter, the centering cement may be used;
but for a lens of a diameter greater than this, sufficient adhesion is
easily obtained with Regnault's mastic, and its low melting point
gives it a decided advantage over the shellac composition.
The glass may be heated gradually by placing it on the water bath, or
actually in the water, and gradually bringing the water up to the
boiling-point. The glass, being taken out, is rapidly wiped, and
rubbed with a bit of waste moistened, not wet, with a little
turpentine: its surface is then rubbed with a stick of mastic
previously warmed so as to melt easily. The surface of the chuck
being also warm, and covered with a layer of melted cement, it is
applied to the glass. The lathe is turned slowly by hand, and the
glass pushed gradually into the most central position; it is then
pressed tight against the chuck by the back rest, a bit of wood being
interposed for obvious reasons.
When all is cold the turning may be proceeded with. The quickest way
is to use the method already described (i.e. actual turning by a file
tool); but if the student prefers (time being no object), he may
accomplish the reduction to a circular form very easily by grinding.
Fig. 45.
Fig. 46.
For this purpose he will require to make the following arrangements
(Fig. 45). If the lathe has a slide rest, a piece of stout iron may
be bent and cut so as to fit the tool rest, and project beneath the
glass. The iron must be fairly rigid, for if it springs appreciably
beneath the pressure of the glass, it will not grind the latter really
round. The lathe may run rather faster than for turning cast iron of
the same size. Coarse emery, passing through a sieve of 80 threads to
the inch (run), may be fed in between the glass and iron, and the
latter screwed up till the disc just grinds slightly as it goes round.
A beginner will generally (in this as in all cases of grinding
processes) tend to feed too fast--no grinding process can be hurried.
If a slide rest is not available, a hinged board, carrying a bit of
iron, may (see Fig. 45) be arranged so as to turn about its hinge at
the back of the lathe; and it may be screwed up readily enough by
passing a long set-screw through the front edge, so that the point of
the screw bears upon the lathe bed. I may add that emery behaves as
if it were greasy, and it is difficult to wet it with clean water.
This is easily got over by adding a little soap or alcohol to the
water, or exercising a little patience.
A good supply of emery and water should be kept between the disc and
the iron; a little putty may be arranged round the point of contact
on the iron to form a temporary trough. In any case the resulting
emery mud should on no account be thrown away, but should be carefully
kept for further use. The process is complete when the glass is
perfectly round and of the required diameter as tested by callipers.
§ 55. The next step is to rough out the lens, and this may easily be
done by rotating it more slowly, i.e. with a surface speed of ten
feet per minute, and turning the glass with a hard file, as explained
in § 42. If it is desired to employ the slide rest, it is quicker and
better to use a diamond tool--an instrument quite readily made, and
of great service for turning emery wheels and the like,--a thing, in
fact, which no workshop should be without. A bit of diamond bort, or
even a clear though off-colour stone, may be employed.
An ordinary lathe tool is prepared by drawing down the tool steel to a
long cone, resembling the ordinary practice in preparing a boring
tool. The apex of the cone must be cut off till it is only slightly
larger than the greatest transverse diameter of the diamond splinter.
The latter may have almost any shape--a triangular point, one side of
a three-sided prism is very convenient. A hole is drilled in the
steel (which must have been well softened), only just large enough to
allow the diamond to enter--if the splinter is thicker in the middle
than at either end, so much the better--the diamond is fastened in
position by squeezing the soft steel walls tightly down upon it.
Personally I prefer to use a tool holder, and in this case generally
mount the diamond in a bit of brass rod of the proper diameter; and
instead of pinching in the sides of the cavity, I tin them, and set
the diamond in position with a drop of soft solder.
Fig. 47.
In purchasing diamond bort, a good plan is to buy fragments that have
been employed in diamond drilling, and have become too small to reset;
in this case some idea as to the hardness of the bits may be obtained.
Full details as to diamond tool-making are given in books on
watch-making, and in Holtzapffell's great work on Mechanical
Manipulation; but the above notes are all that are really
necessary--it is, in fact, a very simple matter. The only advantage of
using a diamond tool for glass turning is that one does not need to be
always taking it out of the rest to sharpen it, which generally happens
with hard steel, especially if the work is turned a little too fast.
I recommend, therefore, that the student should boldly go to work
"free hand" with a hard file; but if he prefer the more formal
method, or distrust his skill (which he should not do), then let him
use a diamond point, even if he has the trouble of making it. When
using a diamond it is not necessary to employ a lubricant, but there
is some advantage in doing so.
The surface of the lens can be roughly shaped by turning to a template
or pattern made by cutting a circular arc (of the same radius as the
required surface) out of a bit of sheet zinc. Another very handy way
of making templates of great accuracy is to use a beam compass
(constructed from a light wooden bar) with a glazier's diamond instead
of a pencil. A bit of thin sheet glass is cut across with this
compass to the proper curvature--which can be done with considerable
accuracy and the two halves of the plate, after breaking along the
cut, are ground together with a view to avoiding slight local
irregularities, by means of a little fine emery and water laid between
the edges. In this process the glass is conveniently supported on a
clean board or slate, and the bits are rubbed backwards and forwards
against each other.
§ 56. It is not very easy for a beginner to turn a bit of
anything--iron, wood, or glass--with great accuracy to fit a template,
and consequently time may be saved by the following procedure, applied as
soon as the figure of the template is roughly obtained. A disc of
lead or iron, of the same diameter as the glass, and of approximately
the proper curvature, is prepared by turning, and is armed with a
handle projecting coaxially from the back of the disc. The glass
revolving with moderate speed on the lathe, the lead tool, supplied
with coarse emery and water, is held against it, care being taken to
rotate the tool by the handle, and also to move it backwards and
forwards across the disc, through a distance, say, up to half an inch;
if it is allowed to overhang too much the edges of the glass disc will
be overground. By the use of such a tool the glass can readily be
brought up to the template.
The only thing that remains, so far as the description of this part of
the process goes, is to give a note or two as to the best way of
making the lead tools, and for this purpose the main narrative of
processes must be interrupted. The easiest way is to make a set of
discs to begin with. For this purpose take the mandrel out of the
lathe, and place it nose downwards in the centre of an iron ring of
proper diameter on a flat and level iron plate.
The discs are made by pouring lead round the screw-nose of the
mandrel. This method, of course, leaves them with a hole in the
centre; but this can be stopped up by placing the hot disc (from
which the mandrel has been unscrewed) on a hot plate, and pouring in a
sufficiency of very hot lead; or, better still, the mandrel can be
supported vertically at any desired distance above the plate while the
casting is being poured. Lead discs prepared in this way are easily
turned so as to form very convenient chucks for brass work, and for
use in the case now being treated, they are easily turned to a
template, using woodturners' tools, which work better if oiled, and
must be set to cut, not scrape.
If the operator does not mind the trouble of cutting a screw, or if he
has a jaw chuck, the lead may be replaced by iron with some advantage.
The following is a neat way of making concave tools. It is an
application of the principle of having the cutting tool as long as the
radius of curvature, and allowing it to move about the centre of
curvature. Place the disc of iron or lead on the lathe mandrel or in
the chuck, and set the slide rest so that it is free to slide up or
down the lathe bed. Take a bar of tool steel and cut it a little
longer than the radius of curvature required. Forge and finish one
end of the bar into a pointed turning tool of the ordinary kind.
Measure the radius of curvature from the point of the tool along the
bar, and bore a hole, whose centre is at this point, through the bar
from the upper to the lower face. I regard the upper face as the one
whose horizontal plane contains the cutting point when the tool is in
use. Clamp a temporary back centre to the lathe bed, and let it carry
a pin in the vertical plane through the lathe centres, and let this
pin exactly fit the hole in the bar.
Fig. 48.
Place the "radius" tool in position for cutting, and let it be lightly
held in the slide rest nearly at the cutting point, the centre of
rotation of the pedestal (or its equivalent) passing through the
central line of the bar. Then adjust the temporary back rest, so that
the tool will take a cut. In the sketch the tool is shown swinging
about the back centre instead of about a pin--there is little to
choose between the methods unless economy of tool steel is an object.
The tool must now be fed across the work. The pedestal must of course
be free to rotate, and the slide rest to slip up and down the bed. In
this way a better concave grinding tool can be made than would be made
by a beginner by turning to a template--though an expert turner would
probably carry out the latter operation so as to obtain an' accuracy
of the same order, and would certainly do it in much less time than
would be required in setting up the special arrangements here
described.
On the other hand, if several surfaces have to be prepared, as in the
making of an achromatic lens, the quickest way would be by the use of
the radius tool, bored of course to work at the several radii
required. I have tried both methods, and my choice would depend
partly on the lathe at my disposal, and partly on the number of
grinding tools that had to be prepared.
Having obtained a concave tool of any given radius, it is easily
copied--negatively, so as to make a convex tool in the following
manner. Adjust the concave tool already made on the back rest, so
that if it rotated about the line of centres, it would rotate about
its axis of figure.
Arrangements for this can easily be made, but of course they will
depend on the detailed structure of the lathe. Use the slide rest as
before, i.e. let it grasp an ordinary turning tool lightly, the
pedestal being fixed, but the rest free to slide up or down the lathe
bed. Push the back rest up till the butt of the turning tool (ground
to a rounded point) rests against the concave grinding tool. If the
diameter of the convex tool required be very small compared with the
radius of curvature of the surface (the most usual case), it is only
necessary to feed the cutting tool across to "copy" the concave
surface sufficiently nearly.
Fig. 49.
There seems no reason, however, why these methods should not be
applied at once to the glass disc by means of a diamond point, and the
rough grinding thus entirely avoided. I am informed that this has
been done by Sir Henry Bessemer, but that the method was found to
present no great advantage in practice. A reader with a taste for
mechanical experimenting might try radius bar tools with small
carborundum wheels rapidly driven instead of a diamond.
Enough has now been said to enable any one to prepare rough convex or
concave grinding tools of iron or lead, and of the same diameter as
the glass to be ground.
The general effect of the process of roughing the rotating lens
surface is to alter the radius of curvature of both tool and glass;
hence it is necessary to have for each grinding tool another to fit
it, and enable it to be kept (by working the two together) at a
constant figure. After a little practice it will be found possible to
bring the glass exactly up to the required curvature as tested by
template or spherometer. The art of the process consists in altering
the shape of the grinding tool so as to take off the glass where
required, as described in § 53, and from this point of view lead has
some advantages; (opinions vary as to the relative advantages of lead
and iron tools for this purpose, however). The subsidiary grinding
tool is not actually needed for this preliminary operation, but it has
to be made some time with a view to further procedure, and
occasionally is of service here.
§ 57. 'The glass disc must be ground approximately to the proper
curvature on each side before any fine grinding is commenced. It is
precisely for this purpose that the previous turning of the disc is
recommended, for it is easy to unmount and recentre a round object,
but not so easy if the object have an indefinite shape. Using a
cement which is plastic before it sets, the disc may be easily taken
off the chuck and centred by a little handicraft, i.e. by rotating
the lathe slowly and pushing the disc into such a position that it
rotates about its axis. The grinding of the second surface is
accomplished exactly as in the former case; of course on reversing
the glass the chuck has to be slightly turned up to fit the convex or
concave surface.
§ 58. There is, however, one point of interest and importance--attention
to which will save a good deal of useless labour afterwards.
The glass must be ground in such a manner that the thickness at the
edge is the same all round. In other words, the axes of figure of the
two surfaces must coincide. This will be the case if the recentering
has been accurately performed, and therefore no pains should be spared
to see that it is exactly carried out. Any simple form of vernier
gauge (such as Brown and Sharpe's vernier callipers) will serve to
allow of a sufficiently accurate measurement of the edge thickness of
the lens. If any difference of thickness is observed as the gauge
moves round the edge, one or other of the surfaces must be reground.
Of course the latitude of error which may be permitted depends so much
on the final arrangements for a special finishing process called the
"centering of the lens"--which will be described--that it is
difficult to fix a limit, but perhaps one-thousandth of an inch may be
mentioned as a suitable amount for a 2-inch disc. For rough work, of
course; more margin may be admitted.
§ 59. In a large shop I imagine that lenses of only two inches
diameter would be ground in nests; or, in other words, a number would
be worked at a time, and centering, even of a rough kind, would be
left to the last; but this process will be treated hereafter. At
present I shall assume that only one lens will be made at a time.
Consequently we now enter on the stage of fine grinding by hand. A
leaden pedestal, for the sake of stability, must be provided on which
to mount the lens, so that the surface to be operated on may be nearly
horizontal (Fig. 50). Before this can be done, however, fresh
grinding tools (two for each surface) must be properly prepared.
After trying several plans I unhesitatingly recommend that all
fine-grinding surfaces should be made of glass. This is easily done
by taking two discs of lead, or iron, or slate, cut to a one-tenth
inch smaller radius of curvature (in the case of a convex tool, and
the opposite in the other case) than the lens surface (Fig. 51, A).
On these, square bits of sheet glass, one-tenth of an inch thick, are
to be cemented, so as to leave channels of about one-eighth of an inch
between each bit of glass (Fig. 52, B). The "mastic" cement
formerly described may be employed for this purpose.
Fig. 50.
The bits of glass ought first to have their edges dressed smooth on
the grind-stone. A convex and concave glass surface having been thus
roughly prepared, they must be mounted in turn in the lathe, and
brought to the proper curvature by grinding with the tools formerly
employed and tested by the template or spherometer. It is well to
control this process by means of a spherometer, so that the desired
radius may be approximately reached. The two glass-grinding tools
are then ground together by hand (see § 53 and § 61), the spherometer
being employed from time to time to check the progress of the work.
In general, if large circular sweeps are taken, greatly overhanging
the side of the glass surface to be figured, both the upper and lower
surfaces will be more ground at the edges, while in the opposite
event the centre will be chiefly affected.
Fig. 51.
A spherometer capable of measuring a 2-inch surface may be procured,
having a screw of, say, 50 threads to the inch, and a micrometer
surface divided into 200 parts, each part easily capable of
subdivision--into tenths or even twentieths. To get the full
advantage of the spherometer it must screw exceedingly freely (i.e.
must be well oiled with clock oil), and must not be fingered except at
the milled head. If one of the legs is held by the fingers the
expansion is sufficient to throw the instrument quite out of
adjustment. The glass-grinding tools being brought to the proper
figure, the next process is to transfer the same to the lens, and this
is done by similar means, the fellow tool being used to correct the
one employed in grinding the lens surface. Before the grade of emery
is changed all three surfaces must agree, as nearly, at least, as the
spherometer will show.
In order to prevent confusion the following summary of the steps
already taken may be given. The discs of glass are first ground or
turned so as to be truly circular. Four "tools" are made for each
surface--a rough pair of iron or lead, and a finishing pair of iron,
lead, or slate faced by glass squares. For a small lens the iron or
lead backing may be used, for a large one the slate. The rough tools
are used to give an approximate figure both to the lens and to the
finishing tools.
The final adjustment is attained by grinding one of the glass-faced
tools alternately upon the lens and upon the fellow glass-faced tool.
The spherometer is accepted at all stages of the process as the final
arbiter as to curvature. Some hints on the form of strokes used in
grinding will be given later on (see § 61). It suffices to state here
that the object throughout is to secure uniformity by allowing both
the work and the tool to rotate, and exercising no pressure by the
fingers. The tool backing may weigh from one to two pounds for a
2-inch lens.
§ 60. The tools and lens being all of the same curvature, the state
of the surface is gradually improved by grinding with finer and finer
emery. The best way of grading the emery is by washing it with clean
water, and allowing the emery (at first stirred up with the water) to
settle out. The longer the time required for this part of the process
the finer will be the emery deposited. An ordinary bedroom jug is a
very good utensil to employ during this process; a large glass jug is
even better. The following grades will be found sufficient, though I
daresay every operative's practice differs a little on this point.
1st grade: Flour emery, with the grit washed out, i.e. allowed to
stand for 2" (sec.) before being poured off.
2nd grade: Stand 5" (secs.), settle in 1' (min.)
3rd grade: Stand 1', settle in 10'.
4th grade: Stand 10', settle in 60'.
It is generally advisable to repeat the washing process with each
grade. Thus, selecting grade 2 for illustration, the liquor for grade
3 must be poured off without allowing any of the sediment to pass over
with it. If any sediment at all passes, one has no security against
its containing perhaps the largest particle in the jug. As soon as
the liquor for No. 3 has been decanted, jug No. 2 is filled up again
with clean water (filtered if necessary), and after standing 5" is
decanted into jug No. 2b, the sediment is returned to jug No. 1, and
the liquor, after standing 1', is transferred to jug No. 3.
The greatest care is necessary at each step of the operation to
prevent "sediment" passing over with liquor. There is a little danger
from the tendency which even comparatively large particles of emery
have to float, in consequence of their refusing to get wet, and the
emery worked up on the side of the jug is also a source of danger,
therefore wipe the jug round inside before decanting.
In order to get a uniform grade stop the currents of water in the jug,
which may work up coarse particles, by holding a thin bit of wood in
the rotating liquid for a moment, and then gently withdrawing it in
its own plane. These precautions are particularly necessary in the
case of grades Nos. 2, 3, and 4, especially No. 4, for if a single
coarse particle gets on the tool when the work has progressed up to
this point it will probably necessitate a return to grinding by means
of No. 2, and involve many hours' work.
The surface of the lens will require to be ground continuously with
each grade till it has the uniform state of roughness corresponding to
the grade in question. Two hours for each grade is about the usual
time required in working such a lens as is here contemplated.
The coarser grades of emery may be obtained by washing ordinary flour
of emery, but the finer ones have to be got from emery which has been
used in the previous processes. It is not a good plan to wash the
finer grades of emery out of the proceeds of very rough grinding say
with anything coarser than flour of emery--as there is a danger of
thereby contaminating the finer grades with comparatively coarse glass
particles (owing to their lightness) and this may lead to scratching.
If the finer grades are very light in colour, it may be inferred that
a considerable portion of the dust is composed of glass, and this does
no good. Consequently time may be saved by stirring up the
light-coloured mass with a little hydrofluoric acid in a platinum
capsule; this dissolves the finely divided glass almost
instantaneously. The emery and excess of hydrofluoric acid may then
be thrown into a large beaker of clean water and washed several times.
Fine emery thus treated has much the same dark chocolate colour as the
coarser varieties.
The operator should not wear a coat, and should have his arms bare
while working with fine emery, for a workshop coat is sure to have
gathered a good deal of dust, and increases the chances of coarse
particles getting between the surfaces.
§ 61. Details of the Process of Fine Grinding.
A lens of the size selected for description is mounted as before
mentioned on a leaden pedestal, and the operator places the latter on
a table of convenient height in a room as free from dust as possible.
Everything should be as clean as a pin, and no splashes of emery mud
should be allowed to lie about. I have found it convenient to spread
clean newspapers on the table and floor, and to wear clean linen
clothes, which do not pick up dust. I have an idea that in large
workshops some simpler means of avoiding scratches must have been
discovered, but I can only give the results of my own experience. I
never successfully avoided scratches till I adopted the precautions
mentioned.
Fig. 52.
The left hand should be employed in rotating the pedestal either
continuously (though slowly) or at intervals of, say, one minute.
This point is rather important. Some operators require two hands to
work the grinding tool, and in any case this is the safer practice.
Under these circumstances the pedestal may be rotated through
one-eighth or tenth of a revolution every three minutes, or
thereabouts. The general motion given to the grinding tool should be
a series of circular sweeps of about one-fourth the diameter of the
glass disc, and gradually carried round an imaginary circle drawn on
the surface of the lens and concentric with it (Fig. 52).
The tool may overhang the lens by a quarter of the diameter of the
latter as a maximum. The circuit may be completed in from twelve to
thirty sweeps. The grinding tool should be lightly held by the
fingers and the necessary force applied parallel to the surface. The
tool itself must be slowly rotated about its axis of figure. If the
tool be lightly held, it will be found that it tends to rotate by
itself. I say "tends to rotate," for if the tool be touching evenly
all over the surface it will rotate in a direction opposite to the
direction of the circular sweep. For instance, if the tool be carried
round its looped path clockwise, it will tend to rotate about its own
axis of figure counter-clockwise. If it touch more in the middle,
this rotation will be increased, while if it touches more along the
edge, the rotation will be diminished, or even reversed in an extreme
case.
Every fifty sweeps or so the tool should be simply ground backwards
and forwards along a diameter of the lens surface. This grinding
should consist of three or four journeys to and fro along, say, eight
different diameters. About one-quarter of the whole grinding should
be accomplished by short straight strokes, during which the tool
should only overhang about one-quarter of an inch. The object of the
straight strokes is to counteract the tendency to a gradual
accumulation of the emery in the centre, which results from the
circular grinding.
A great deal of the art of the process consists in knowing how to work
the tool to produce any given effect. For instance, if the lens
requires to be ground down near the centre, the epicycloidal strokes
must be nearly central; the tool must never overhang very much. If,
on the other hand, it is the edges which require attention, these must
be dealt with by wider overhanging strokes. The tool must be
frequently tested on its fellow, and, indeed, ground upon it if any
marked unevenness of action (such as that just described) is required
for the lens. A check by spherometer will be applied at intervals
according to the judgment of the operator, but, in any case, the
fellow tool and lens should be kept at very nearly the same figure.
The emery should never be allowed to become anything like dry between
the tool and the lens, for in some way (probably by capillary action
increasing the pressure of the tool) this seems to lead to scratching
and "rolling" of the emery. The channels in the glass tool between
the squares are of the greatest importance in enabling the emery to
distribute itself. Perhaps the best guide in enabling one to judge
as to when it is time to wash off the emery and apply fresh is the
"feel" of the tool; also when the mud gets light in colour we know
that it is full of glass dust, and proportionately inoperative.
New emery may be put on, say, every five minutes, but no absolute rule
can be given, for much depends on the pressure of the tool upon the
lens. In the case considered a brass or lead, or even slate tool, of
an inch, or even less, in thickness, will press quite heavily enough.
In washing the lens and tool before new emery is introduced, a large
enamelled iron bucket is very handy; the whole of the tool should be
immersed and scrubbed with a nail-brush. The lens surface may be
wiped with a bit of clean sponge, free from grit, or even a clean damp
cloth.
When the time comes to alter the grade of emery, a fresh lot of
newspapers should be put down, and tools, lens, and pedestal well
washed and brushed by the nail-brush. The surfaces should be wiped
dry by a fresh piece of rag, and examined for scratches and also for
uniformity of appearance; a good opinion can be formed as to the fit
of the surfaces by noting whether--and if so, to what degree--they
differ in appearance from point to point when held so that the light
falls on them obliquely.
It is necessary to exercise the greatest care in the washing between
the application of successive grades of emery, and this will be
facilitated if the edges of the glass squares were dressed on a
grindstone before they were mounted. An additional precaution which
may be of immense advantage is to allow the tool to dry between the
application of successive grades of emery (of course, after it has
been scrubbed), and then to brush it vigorously with a hat-brush. It
sometimes happens that particles of mud which have resisted the wet
scrubbing with the nail-brush may be removed by this method.
As my friend Mr. Cook informs me that his present practice differs
slightly from the above, I will depart from the rule I laid down, and
add a note on an alternative method.
Consider a single lens surface. This is roughed out as before by an
iron tool, a rough fellow tool being made at the same time. The
squares of glass are cemented to the roughing tool, and this is ground
to the spherometer by means of the counterpart tool. The glass-coated
tool is then applied to the lens surface and grinding with the first
grade of emery commenced. The curvature is checked by the
spherometer. Two auxiliary tools of, say, half the diameter of the
lens, are prepared from slate, or glass backed with iron, and applied
to grind down either the central part of the lens surface or tool
surface, according to the indications of the spherometer. Any changes
that may occur during grinding are corrected by these tools. The
spherometer is accepted as the sole guide in obtaining the proper
curvature. A slate backing is preferred for tools of any diameter
over, say, 2 inches.
§ 62. Polishing.
After the surface has been ground with the last grade of emery, and
commences to become translucent even when dry, the grinding may be
considered to be accomplished, and the next step is the polishing.
There are many ways of carrying out this process, and the relative
suitability of these methods depends on a good many, so to speak,
accidental circumstances. For instance, if the intention is to finish
the polishing at a sitting, the polishing tool may be faced with
squares of archangel--not mineral or coal-tar--pitch and brought to
shape simply by pressing while warm against the face of the lens. A
tool thus made is very convenient, accurate, and good, but it is
difficult to keep it in shape for any length of time; if left on the
lens it is apt to stick, and if it overhangs ever so little will, of
course, droop at the edges.
On the whole, the following will be found a good and sufficient plan.
The glass-grinding tool is converted into a polishing tool by pasting
a bit of thin paper over its surface; a bit of woven letter paper of
medium thickness with a smooth but not glazed surface does very well.
We have found that what is called Smith's "21 lbs. Vellum Wove" is
excellent. This is steeped in water till quite pliable and almost
free from size. The glass tool is brushed over with a little thin
arrowroot or starch paste, and the paper is laid upon it and squeezed
down on the glass squares as well as possible; if the paper is wet
enough and of the proper quality it will expand sufficiently to
envelop the tool without creases, unless the curvature is quite out of
the common.
This being accomplished, and the excess of water and paste removed,
the face of the paper is (for security) washed with a little clean
water and a bit of sponge, and, finally, the tool is slightly pressed
on the lens so as to get the paper to take up the proper figure as
nearly as possible. After the polishing tool has been thus brought to
the proper figure, it is lifted off and allowed to dry slowly. When
the paper is dry it may be trimmed round the edges so as not to
project sensibly beyond the glass squares. The next step is to brush
the surface over very carefully with polishing rouge (prepared as is
described at the end of this section) by means of a hat-brush. When
the surface of the paper is filled with rouge all excess must be
removed by vigorous brushing.
Fig. 53.
The tool being placed on the lens, two or three strokes similar to
those used in grinding may be taken, and the tool is then lifted off
and examined. It will be found to be dotted with a few bright points,
produced by the adhesion of glass at the places of contact. These
points are then to be removed in the following manner. An old
three-cornered file is ground on each side till the file marks
disappear, and sharp edges are produced (Fig. 53). This tool is used
as an ink eraser, and it will be found to scrape the paper of the
polishing tool very cleanly and well.
The bright spots are the objects of attention, and they must be erased
by the old file, and the polisher reapplied to the glass. A few
strokes will develop other points, more numerous than before, and
these in turn must be erased. The process is continued till the whole
surface of the polishing tool is evenly covered with bright specks,
and then the polishing may be proceeded with. The specks should not
be more than about one-eighth of an inch apart, or the polishing will
be irregular.
The operation of polishing is similar to that of grinding. A
reasonable time for polishing a glass surface is twenty hours; if
more time is required it is a sign that the fine grinding has not been
carried far enough. The progress of the operation may be best watched
by looking at the surface--not through it. For this purpose a good
light is requisite. When the lens is dismounted it may be examined by
a beam of sunlight in a dark room, under which circumstances the
faintest signs of grayness are easily discernible.
It may be mentioned here that if the surface is in any way scratched
the rouge will lodge in the scratches with great persistence, and an
expert can generally tell from the appearance of scratches what kind
of polishing powder has been employed.
The persistence with which rouge clings to a rough surface of glass is
rather remarkable. Some glass polishers prefer to use putty powder as
a polishing material, and it is sometimes said to act more quickly
than rouge; from my rather limited experience I have not found this
to be the case, but it may have merits that I do not know of. Is it
possible that its recommendation lies in the fact that it does not
render scratches so obtrusively obvious as rouge does?
Rouge is generally made in two or more grades. The softer grade is
used for polishing silver, and is called jewellers' rouge. The harder
grade, suitable for glass polishing, is best obtained from practical
opticians (not mere sellers of optical instruments). I mean people
like Messrs. Cook of York. Many years ago I prepared my own hard
rouge by precipitating ferrous sulphate solution by aqueous ammonia,
washing the precipitate, and heating it to a red heat. The product
was ground up with water, and washed to get rid of large particles.
This answered every purpose, and I could not find that it was in any
way inferior to hard rouge as purchased. The same precipitate heated
to a lower temperature is said to furnish a softer variety of rouge;
at all events, it gives one more suitable for polishing speculum
metal. Lord Rosso used rouge heated to a dull redness for this
purpose.
Rouge, whether made or bought, should always be washed to get rid of
grit. I ought to add that not the least remarkable fact about the
polishing is the extraordinarily small quantity of the polishing
material requisite, which suggests that the process of polishing is
not by any means the same as that of exceptionally fine grinding. Is
it possible that the chief proximate cause of the utility of rouge is
to be sought in its curious property of adhering to a rough glass
surface, causing it, so to speak, to drag the glass off in minute
quantities, and redeposit it after a certain thickness has been
attained on another part of the surface?
§ 63. Centering.
When a lens is ground and polished it will almost always happen that
the axis of revolution of its cylindrical edge is inclined to the axis
of revolution of its curved surfaces. Since in practice lenses have
to be adjusted by their edges, it is generally necessary to adjust the
edge to a cylinder about the axis of figure of the active surfaces.
This is best done on a lathe with a hollow mandrel.. The lens is
chucked on a chuck with a central aperture--generally by means of
pitch or Regnault's mastic, or "centering" cement for small
lenses--and a cross wire is fixed in the axis of revolution of the lathe,
and is illuminated by a lamp. This cross wire is observed by an
eye-piece (with cross wires only in the case of a convex lens, or a
telescope similarly furnished in the case of a concave lens),
also placed in the axis of rotation of the lathe.
Both cross wires are thus in the axis of revolution of the mandrel,
and the distant one (B in the figure) is viewed through the lens and
referred to the fixed cross wires at A. In general, as the lathe is
rotated by turning the mandrel the image of the illuminated cross
wires will be observed to rotate also. The lens is adjusted until the
image remains steady on rotating the mandrel and it is to give time
for this operation that a slow-setting cement is recommended. When
the image remains stationary we know that the optical centre of the
lens is in the axis of revolution, and that this axis is normal to
both lens surfaces, i.e. is the principal axis of the lens, or axis
of figure.
Fig. 54.
A much readier method, and one, in general, good enough for most
purposes, is to put a candle on the end of the lathe-bed where the
back centre generally is, and observe the images of the flame by
reflection from both the lens surfaces. This method is very handy
with small lenses; the mandrel is turned, and the lens adjusted by
hand till the images are immovable. In both cases, of course, the
edge of the lens is turned or ground till it is truly circular, the
position of the lens remaining undisturbed on the chuck. If the edge
gauge has been properly used in the earlier stages of figuring, it
will be found that very little turning or grinding is requisite to
produce a true centering.
The particular defect due to want of centering in a lens may be
observed by using it as the objective of a telescope, and observing a
star slightly out of focus. The interference fringes will not be
concentric circles unless the lens is properly centred. I ought to
say that I have not looked into the theory of this, but have merely
taken it as a generally admitted fact. The diseases of lenses and the
modes of treating them are dealt with in a book by Messrs. Cook of
York, entitled On the Adjustment and Testing of Telescopic Objectives.
The final process of figuring will be dealt with later on (§ § 66 and
67), as it applies not only to lenses but to mirrors, prisms, etc. If
the instructions given have been carefully carried out on a 2-inch
lens, it should perform fairly well, and possibly perfectly, without
any further adjustment of the glass.
§ 64. Preparation of Small Lenses, where great Accuracy is not of the
first Importance.
Such lenses may generally be made out of bits of good plate or sheet
glass, and are of constant use in the physical laboratory. They may
be purchased so cheaply, however, that only those who have the
misfortune to work in out-of-the-way places need be driven to make
them.
Suitable glass having been obtained and the curves calculated from the
index of refraction, as obtained by any of the ordinary methods
applicable to plates (the microscope method, in general, is quite good
enough), squares circumscribing the desired circles are cut out by the
help of a diamond. [Footnote: Glazebrook and Shaw's Practical
Physics, p. 383 (4th ed.).] The squares are roughly snipped by means
of a pair of pliers or spectacle-maker's shanks. The rough circles
are then mounted on the end of a brass or iron rod of rather greater
diameter than the finished lenses are to possess. This mounting is
best done by centering cement.
The discs are then dressed circular on a grindstone, the rod serving
both as a gauge and handle. A sufficient number of these discs having
been prepared, a pair of brass tools of the form shown in the sketch
(Fig. 55), and of about the proper radius of curvature, are made.
One of these tools is used as a support for the glass discs.
Fig. 55.
A compass being set to scribe circles of the same diameter as the
glass discs, centre marks are made on the surface of the appropriate
tool, circles are drawn on this, and facets are filed or milled (for
which the spiral head of the milling machine is excellent). In the
case of concave supporting surfaces, i.e. in making concave lenses, I
apprehend filing would be difficult, and the facets would have to be
made by a rose cutter or mill; but if the discs are fairly round,
then, in fact, no facets are required.
The facets being ready, the glass discs are cemented to them by
centering cement, which may be used quite generally for small lenses.
When the cutting of facets has been omitted on a concave surface, the
best cement is hard pitch. The grinding tool is generally rather
larger than the nest of lenses. Coarse and fine grinding is
accomplished wholly on the lathe--the tool being rotated at a fair
speed (see infra), and the nest of lenses moved about by its handle so
as to grind all parts equally. It must, of course, be held anywhere
except "dead on," for then the part round the axis would not get
ground; this inoperative portion of the rotating tool must therefore
be allowed to distribute its incapable efforts evenly over the nest of
lenses.
Polishing is accomplished by means of the grinding tool, coated with
paper and rouge as before; or the tool may be coated with very thin
cloth and used with rouge as before--in this case the polishing goes
on fastest when the surface of the cloth is distinctly damp. In
working by this method, each grade of emery need only be applied from
five to ten minutes. The glass does not appear to get scratched when
the emery is changed, provided everything is well washed. A good
polish may be got in an hour. The lathe is run as for turning brass
of the same diameter as the tool.
One side of the lenses being thus prepared, they are reversed, and the
process gone through for the other side in a precisely similar manner.
[Footnote: Unless the radius of curvature is very short and the lenses
also convex, there is no necessity to recess the facets, provided hard
pitch is used as the cement. See note on hard pitch.] To save
trouble, it is usual, to make such lenses of equal curvature on both
faces; but of course this is a matter of taste.
Fig. 56.
For very common work, bits of good plate glass are employed, and the
manufacturer's surface treated as flat (Fig. 56). In this way
plano-convex lenses are easily and cheaply made. Finally the lenses
have to be centred, an essential operation in this case. This is
easily done by the reflection method--the edge being turned off by
the file and kerosene and the centering cement being used in making
the preliminary adjustment on the chuck. I presume a lens made in
this way is worth about a shilling, so that laboratory manufacture is
not very remunerative. Fig. 56 shows the method of mounting small
lenses for lathe grinding, when only one lens is required. The tool
is generally rotated in the lathe and the lens held against it.
§ 65. Preparing Small Mirrors for Galvanometers.
To get good mirrors for galvanometers, I have found the best plan is
to grind and polish a large number together, on a disc perhaps 8 or 10
inches in diameter. I was led to this after inspecting and rejecting
four ounces of microscope cover slips, a most wearisome process. That
regular cover slips should be few and far between is not unlikely,
seeing that they are made (by one eminent firm at least) simply by
"pot" blowing a huge thin bulb, and then smashing it on the floor and
selecting the fragments. As in the case of large mirrors, it is of
course only necessary to grind one side of the glass, theoretically at
all events. The objections to this course are:
(1) A silver surface cannot, in my experience, be polished externally
(on a minute object like a cover slip) to be anything like so bright
as the silver surface next the glass; and,
(2) if one side only is ground, it will be found that the little
mirror hopelessly loses its figure directly it is detached from the
support on which it has been worked. Consequently, I recommend that
these small mirrors should be ground and polished on both
sides--enough may be made at one operation to last for a very long time.
A slate back is prepared of the same radius of curvature as it is
desired to impart to the mirrors. Bits of thin sheet glass are then
ground circular as described in the last section and cemented to this
surface by the smallest quantity of clean archangel pitch, allowed to
cool slowly and even to rest for a day before the work is proceeded
with. The whole surface is then ground and polished as before.
The mirrors are now reversed, when they ought to nearly fit the tool
(assuming that flats are being made, and the fellow tool in all other
cases), and are recemented by pitch to the appropriate backing ground,
and polished. If very excellent results are required, these processes
may be preceded by a preliminary rough grinding of one surface, so
that the little discs will "sit" exactly on the tool surface, and not
run the risk of being strained by capillary forces in the pitch. We
have always found this necessary for really good results.
On removing such mirrors from the backing, they generally, more or
less lose their figure, becoming (in general fairly uniformly) more
concave or convex. About 5 per cent of the mirrors thus prepared will
be found almost perfect if the work has been well done, and the rest
will probably be very fair, unless the diameter is very large as
compared with the thickness. The best way of grinding and polishing
such large surfaces (nests 10 inches in diameter) is on a grinding
machine, such as will be described below. The polishing is best done
by means of paper, as before described.
Having occasion to require hitherto unapproached lightness and optical
accuracy in such mirrors, I got my assistant to try making them of
fused quartz, slices being cut by a diamond wheel from a rod of that
material. Chips of natural quartz were also obtained from broken
"pebble" spectacles, and these were worked at the same time. The
resulting mirrors were certainly superior to the best we could make
from glass, but the labour of grinding was greater, and the labour of
polishing less, than in the latter case. The pebble fragments gave
practically as good mirrors as the fused slices. For the future it
will be better always to make galvanometer mirrors from quartz
crystals. These may be easily sliced, as will be described in § 74.
The slices are dressed on a grindstone according to instructions
already given for small lenses.
The silvering of these mirrors is a point of great importance. After
trying nearly every formula published, we have settled down to the
following.
A solution of pure crystallised nitrate of silver in distilled water
is made up to a strength of 125 grams of the salt per litre. This
forms the stock solution and is kept in a dark bottle.
Let the volume of silvering liquor required in any operation be
denoted by 4 v. The liquor is prepared as follows:
I. Measure out a volume v of the stock solution of silver nitrate,
and calculate the weight of salt which it contains; let this be w.
In another vessel dissolve pure Rochelle salt to the amount of 2.6 w,
and make up the solution to the volume v. These two solutions are to
be mixed together at a temperature of 55° C, the vessels with their
contents being heated to this temperature on the water bath. After
mixing the liquids the temperature is to be kept approximately
constant for five minutes, after which the liquor may be cooled. The
white precipitate which first forms will become gray or black and very
dense as the liquid cools. If it does not, the liquor must be
reheated to 55° C, and kept at that temperature for a few minutes and
then again allowed to cool. The solution is in good order when all
the precipitate is dense and gray or black and the liquor clear. The
blacker and denser the precipitate the better is the solution. The
liquor is decanted and filtered from the precipitate and brought up to
the volume 2 v by addition of some of the wash water.
II. Measure out a volume 0.118 v of the stock solution into a
separate vessel, and add to it a 5 per cent solution of ammonium
hydrate, with proper precautions, so that the precipitate at first
formed is all but redissolved after vigorous shaking. It is very
important that this condition should be exactly attained. Therefore
add the latter part of the ammonia very carefully. Make up the volume
to 2 v.
Mix the solutions I. and II. in a separate vessel and pour the
mixture into the depositing vessel. The surface to be silvered should
face downwards, and lie just beneath the free surface of the liquid.
Bubbles must of course be removed.
The silver deposit obtained in this manner is exceedingly white and,
bright on the surface next to the glass, but the back is mat and
requires polishing.
The detail of the process described above was worked out in my
laboratory by Mr. A. Pollock, to whom my thanks are due.
This process gives good deposits when the solutions are freshly
prepared, but the ammonia solution will not keep; The surfaces to be
silvered require to be absolutely clean. The process is assisted by a
summer temperature, say 70° Fahr, and possibly by the action of
light. Six or seven hours at least are required for a good deposit; a
good plan is to leave the mirrors in the bath all night. On removal
from the bath the mirrors require to be well washed, and allowed to
dry thoroughly in sun heat for several hours before they are touched.
Care should be taken not to pull the mirrors out of shape when they
are mounted for the bath. A single drop of varnish or paint (a mere
speck) on the centre will suffice to hold them. The back of the
deposit requires to be varnished or painted as a rule to preserve the
silver. All paints and varnishes thus applied tend to spoil the
figure by expanding or contracting. On the whole, I think boiled
linseed oil and white or red lead--white or red paint in fact--is
less deleterious than other things I have tried. Shellac varnish is
the worst.
Of course, the best mirror can be easily spoiled by bad mounting. I
have tried a great number of methods and can recommend as fairly
successful the following:- A little pure white lead, i.e. bought as
pure as a chemical--not as a paint--is mixed with an equal quantity
of red lead and made into a paste with a little linseed oil. I say a
paste, not putty. A trace of this is then worked on to the back of
the mirror at the centre as nearly as may be, and to this is attached
the support. The only objection to this is that nearly a week is
required for the paste to set. If people must use shellac let it be
remembered that it will go on changing its shape for months after it
has cooled (whether it has been dissolved in alcohol or not).
§ 66. Preparation of Large Mirrors or Lenses for Telescopes.
So much has been written on this subject by astronomers, generally in
the English Mechanic and in the Philosophical Transactions for 1840,
that it might be thought nothing could be added. I will only say here
that the processes already described apply perfectly to this case; but
of course I only refer to silver on glass mirrors. For any size over
6 inches in diameter, the process of grinding and polishing by hand,
particularly the latter, will probably be found to involve too much
labour, and a machine will be required. A description of a
modification of Mr. Nasmyth's machine--as made by my assistant, Mr.
Cook--will be found below.
There is no difficulty in constructing or working such a machine, and
considered as an all round appliance, it possesses solid advantages
over the simple double pulley and crank arrangement, which, however,
from its simplicity deserves a note. Two pulleys, A and B, of about
18 inches diameter by 4 inches on the face, are arranged to rotate
about vertical axes, and belted together. The shaft of one of these
pulleys is driven by a belt in any convenient manner. Each pulley is
provided on its upper surface with a crank of adjustable length
carrying a vertical crank-pin.
Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6" long, and
these rods are pinned together at their farther extremities, and this
pin carries the grinding or polishing tool, or rather engages loosely
with the back of this tool which lies below the rod. It is clear that
if the pulleys are of commensurable diameters, and are rigidly
connected--say by belting which neither stretches nor slips--the
polishing tool will describe a closed curve. If, however, the belt is
arranged to slip slightly, or if the pulleys are of incommensurable
diameters, the curve traced out by the grinding tool will be very
complex, and in the case of the ratio of the diameters being
incommensurable, will always remain open; for polishing purposes the
consummation to be wished.
Mirror surfaces are ground spherical, the reduction to parabolic form
being attained in the process of polishing. A very interesting
account of the practice of dealing with very large lenses will be
found in Nature, May 1886, or the Journal of the Society of Arts, same
date (I presume), by Sir Howard Grubb. The author considers that the
final adjustment of surfaces by "figuring"--of which more anon--is
an art which cannot be learned by inspection, any more than a man
could learn to paint by watching an artist. This is, no doubt, the
case to some extent; still, a person wishing to learn how to figure a
lens could not do better than take Sir Howard at his word, and spend a
month at his works. Meanwhile the following remarks must suffice; it
is not likely that anybody to whom these notes will be of service
would embark on such large work as is contemplated by Sir Howard
Grubb.
Fig. 57.
Description of Polishing Machine. Power is applied through belting to
the speed cone A. By means of a bevel pinion rotation is communicated
to the wheel D, which is of solid metal and carries a T-slot, C. A
pedestal forming a crank-pin can be clamped so as to have any desired
radius of motion by the screw E. A train of wheels E F G H K
(ordinary cast lathe change wheels) communicate any desired ratio of
motion to the tool-holder, which simply consists of two pins
projecting vertically downwards from the spokes of wheel K.
These pins form a fork, and each prong engages in a corresponding hole
in the back of the slate-grinding tool (not shown in figure). The
connection with the tool is purposely loose. The wheel E, of course,
cannot rotate about the crank-pin D. Provision for changing the ratio
of tool rotation is achieved by mounting the wheels composing the
train on pins capable of sliding along a long slot in the bar
supporting them. The farther end of this bar is caused to oscillate
to and fro very slowly by means of an additional crank-pin S and
crank-shaft, the projecting face of the bed-plate W being placed
so as to allow V to slide about easily and smoothly.
Motion is communicated to this part of the system by means of
gears at 0 and P, and a belt working from P to Q.
Thus the vertical shaft R is set in motion and
communicates by gears with S. A pulley placed on the axle of the
wheel carrying the crank-pin S gives a slow rotation to the work which
is mounted on the table M. A small but important feature is the tray L
below the gear K. This prevents dirt falling from the teeth of the
wheel on to the work. The motion of S is of course very much less
than of B--say 100 times less. The work can be conveniently adjusted
as to height by means of the screw N.
The machine must be on a steady foundation, and in a place as free
from dust as possible. Though it looks complicated it is quite
straight-forward to build and to operate.
It is explained in Lord Rayleigh's article on Optics in the
Encyclopaedia Britannica that a very minute change in the form of the
curvature of the surface of a lens will make a large difference in the
spherical aberration. This is to be expected, seeing that spherical
aberration is a phenomenon of a differential sort, i.e. a measure of
the difference between the curvature actually attained, and the
theoretical curvature at each point of the lens, for given positions
of point and image. Sir H. Grubb gives an illustration of the
minuteness of the abrasion required in passing from a curve of one
sort to a curve of another, say from a spherical to a parabolic curve,
consequently the process of figuring by the slow action of a polishing
tool becomes quite intelligible. In making a large mirror or lens all
the processes hitherto described under grinding and polishing, etc,
have to be gone through and in the manner described, and when all this
is accomplished the final process of correcting to test commences.
This process is called figuring.
§ 67. Of the actual operation of this process I have no personal
knowledge, and the following brief notes are drawn from the article by
Sir H. Grubb, from my assistant's (Mr. Cook) experience, and from a
small work On the Adjustment and Testing of Telescopic Objectives, by
T. Cook and Sons, Buckingham Works, York (printed by Ben Johnson and
Co, Micklegate, York). This work has excellent photographs of the
interference rings of star images corresponding to various defects.
It must be understood that the following is a mere sketch. The art
will probably hardly ever be required in laboratory practice, and
those who wish to construct large telescopes should not be above
looking up the references.
The process is naturally divided for treatment into two parts.
(1) The detection of errors, and the cause of these errors.
(2) The application of a remedy.
(1) A lens, being mounted with its final adjustments, is turned on to
a star, which must not be too bright, and should be fairly overhead.
The following appearances may be noted:-
A. In focus, the star appears as a small disc with one or two rings
round it; inside and outside of the focus the rings increase in
number, are round, concentric with the disc, and the bright and dark
rings are apparently equally wide. The appearance inside the focus
exactly resembles that outside when allowance is made for chromatic
effects. Conclusion: objective good, and correctly mounted.
B. The rings round the star in focus are not circular, nor is the
star at the centre of the system. In bad cases the fringes are seen at
one side only. Effects exaggerated outside and inside the focus.
Conclusion: the lens is astigmatic, or the objective is not adjusted
to be co-axial with the eyepiece.
C. When in focus the central disc is surrounded by an intermittent
diffraction pattern, i.e. for instance the system of rings may appear
along, and near, three or more radii. If these shift when the points
of support of the lens are shifted, flexure may be suspected.
D. On observing inside and outside the focus, the rings are not
equally bright and dark. This may be due to uncorrected spherical
aberration, particularly to a fault known as "zonal aberration," where
different zones of the lens have different foci, but each zone has a
definite focus.
E. Irregular diffraction fringes point to bad annealing of the glass.
This may be checked by an examination of the lens in polarised light.
F. If the disc appear blurred and coloured, however the focus be
adjusted, incomplete correction for chromatic aberration is inferred.
If in addition the colouring is unsymmetrical (in an extreme case the
star disc is drawn out to a coloured band), want of centering is to be
inferred. This will also show itself by the interference fringes
having the characteristics described in C.
(2) The following steps may be taken in applying a remedy:
A. The adjusting screws of the cell mounting the object glass may
be worked until the best result is attained; this requires great
care and patience. Any errors left over are to be attributed to
other causes than the want of collinearity of the axes of object
glass and eyepiece.
B. Astigmatism is detected by rotating the object glass or object
glass cell. If the oval fringes still persist and the longer axis
follows the lens, astigmatism may be inferred. Similarly, by rotating
one lens on the other, astigmatism, or want of centering (quite a
different thing) may be localised to the lens.
C. The presence of flexure may be confirmed by altering the position
of the points of support with respect to the eyepiece, the lens
maintaining its original position. The addition of more points of
support will in general reduce the ill effects. How to get rid of
them I do not know; they are only serious with large lenses.
D. Spherical aberration may be located by using stops and zonal
screens, and observing the effect on the image. Sir H. Grubb
determines whether any point on the lens requires to be raised or
lowered, by touching the glass at that point with a warm hand or
cooling it by ether. The effects so produced are the differential
results of the change of figure and of refractive index. By observing
the effect of the heating or cooling of any part, the operator will
know whether to raise or lower that part, provided that by a suitable
preliminary experiment he has determined the relation between the
effect produced by the change of figure, and that due to the
temperature variation of the refractive index. In general it is
sufficient to consider the change of shape only and neglect the change
in refractive power.
E. Marked astigmatism has never been noticed by me, but I should
think that the whole lens surface would require to be repolished or
perhaps reground in this case.
F. To decide in which surface faults exist is not easy. By placing a
film of oil between the two surfaces nearly in contact these may be
easily examined. Thus a mixture of nut and almond oil of the right
proportion, to be found by trial, for the temperature, will have the
same refractive index as the crown glass, and will consequently reduce
any errors of figure in the interior crown surface, if properly
applied between the surfaces. Similarly the interior of the flint
surface may have its imperfections greatly reduced in effect by using
almond oil alone, or mixed with bisulphide of carbon. The outer
surfaces, I presume, must be examined by warming or cooling over
suitable areas or zones.
The defects being detected, a matter requiring a great deal of skill
and experience according to Sir H. Grubb, the next step is to remedy
them; and the remedial measures as applied to the glass constitute the
process of figuring. There are two ways of correcting local defects,
one by means of small paper or pitch covered tools, which according to
Sir H. Grubb is dangerous, and according to the experience of Mr.
Cook, and I think of many French opticians, safe and advantageous.
Pitch polishing tools are generally used for figuring. They are made
by covering a slate backing with squares of pitch. The backing is
floated with pitch say one-eighth of an inch thick. This is then
scored into squares by a hot iron rod. The tool, while slightly warm,
is laid upon the lens surface, previously slightly smeared with dilute
glycerine, until the pitch takes the figure of the glass. The
polishing material is rouge and water. Small tools are applied
locally, and probably can only be so applied with advantage for grave
defects.
The other method is longer and probably safer. It consists in
furnishing the polishing tool with squares of pitch as before. These
being slightly warm, the lens is placed upon them so that they will
flow to the exact figure also as before. I presume that the lens is
to be slightly smeared with glycerine, or some equivalent, to keep the
pitch from sticking. The squares are most thickly distributed where
the abrasion is most required, i.e. less pitch is melted out by the
iron rod. This may be supplemented by taking advantage of differences
of hardness of pitch, making some squares out of harder, others out of
softer pitch. The aim is to produce a polishing tool which will
polish unequally so as to remove the glass chiefly from predetermined
parts of the lens surface. The tool is worked over the surface of the
lens by the polishing machine, and part of the art consists in
adjusting the strokes to assist in the production of the local
variations required.
A source of difficulty and danger lies in the fact that the pitch
squares are rarely of the same hardness, so that some abrade the glass
more rapidly than others. This is particularly likely to occur if the
pitch has been overheated. [Footnote: When pitch is heated till it
evolves bubbles of gas its hardness increases with the duration of the
process.] The reader must be good enough to regard these remarks as
of the barest possible kind, and not intended to convey more than a
general idea of the nature of the process of figuring.
§ 68. A few remarks on cleaning lenses will fittingly close this part
of the subject. There is no need to go beyond the following
instructions given by Mr. Brashear in Popular Astronomy, 1894, which
are reproduced here verbatim.
"The writer does not advise the use of either fine chamois skin,
tissue paper, or an old soft silk handkerchief, nor any other such
material to wipe the lenses, as is usually advised. It is not,
however, these wiping materials that do the mischief, but the dust
particles on the lenses, many of them perhaps of a silicious nature,
which are always harder than optical glass, and as these particles
attach themselves to the wiping material they cut microscopic or
greater scratches on the surfaces of the objective in the process of
wiping.
"I write this article with the hope of helping to solve this
apparently difficult problem, but which in reality is a very simple
one.
"Let us commence by taking the object glass out of its cell. Take out
the screws that hold the ring in place, and lift out the ring.
Placing the fingers of both hands so as to grasp the objective on
opposite sides, reverse the cell, and with the thumbs gently press the
objective squarely out of the cell on to a book, block of wood, or
anything a little less in diameter than the objective, which has had a
cushion of muslin or any soft substance laid upon it. One person can
thus handle any objective up to 12 inches in diameter.
"Before separating the lenses it should be carefully noted how they
were put together with relation to the cell, and to one another, and
if they art not marked they should be marked on the edges
conspicuously with a hard lead pencil, so that when separated they may
be put together in the same way, and placed in the same relation to
the cell. With only ordinary precaution this should be an easy
matter.
"Setting the objective on edge the two lenses may be readily
separated.
"And now as to the cleaning of the lenses. I have, on rare occasions,
found the inner surfaces of an object glass covered with a curious
film, not caused directly by moisture but by the apparent oxidation of
the tin-foil used to keep the lenses apart. "A year or more ago a
7-inch objective made by Mr. Clark was brought to me to clean. It had
evidently been sadly neglected. The inside of the lenses were covered
with such a film as I have mentioned, and I feared the glass was
ruined. When taken apart it was found that the tin-foil had oxidised
totally and had distributed itself all over the inner surfaces. I
feared the result, but was delighted to find that nitric acid and a
tuft of absorbent cotton cut all the deposit off, leaving no stains
after having passed through a subsequent washing with soap and water.
"I mention this as others may have a similar case to deal with.
"For the ordinary cleaning of an objective let a suitable sized
vessel, always a wooden one, be thoroughly cleaned with soap and
water, then half filled with clean water about the same temperature as
the glass. Slight differences of temperature are of no moment. Great
differences are dangerous in large objectives.
"I usually put a teaspoonful of ammonia in half a pail of water, and
it is well to let a piece of washed 'cheese cloth' lie in the pail, as
then there is no danger if the lens slips away from the hand, and, by
the way, every observatory, indeed every amateur owning a telescope,
should have plenty of 'cheese cloth' handy. It is cheap (about 3 cts.
per yard) and is superior for wiping purposes to any 'old soft silk
handkerchief,' chamois skin, etc. Before using it have it thoroughly
washed with soap and water, then rinsed in clean water, dried and laid
away in a box or other place where it can be kept clean. When you use
a piece to clean an objective throw it away, it is so cheap you can
afford to do so.
"If the lenses are very dirty or 'dusty,' a tuft of cotton or a
camel's-hair brush may be used to brush off the loose material before
placing the lenses in the water, but no pressure other than the weight
of the cotton or brush should be used. The writer prefers to use the
palms of the hand with plenty of good soap on them to rub the
surfaces, although the cheese cloth and the soap answers nicely, and
there seems to be absolutely no danger of scratching when using the
hands or the cheese cloth when plenty of water is used; indeed when I
wish to wipe off the front surface of an objective in use, and the
lens cannot well be taken out, I first dust off the gross particles
and then use the cheese cloth with soap and water, and having gone
over the surface gently with one piece of cloth, throw it away and
take another, perhaps a third one, and then when the dirt is, as it
were, all lifted up from the surface, a piece of dry cheese cloth will
finish the work, leaving a clean brilliant surface, and no scratches
of any kind.
"In washing large objectives in water I generally use a 'tub' and
stand the lenses on their edge. When thoroughly washed they are taken
out and laid on a bundle of cheese cloth and several pieces of the
same used to dry them.
"I think it best not to leave them to drain dry; better take up all
moisture with the cloth, and vigorous rubbing will do no harm if the
surfaces have no abrading material on them. I have yet to injure a
glass cleaned in this way.
"This process may seem a rather long and tedious one, but it is not so
in practice, and it pays.
"In some places objectives must be frequently cleaned, not only
because they become covered with an adherent dust, but because that
dust produces so much diffused light in the field as to ruin some
kinds of telescope work. Mr. Hale of the Kenwood Observatory tells me
he cannot do any good prominence photography unless his objective has
a clean surface; indeed every observer of faint objects or delicate
planetary markings knows full well the value of a dark field free from
diffused light. The object-glass maker uses his best efforts to
produce the most perfect polish on his lenses, aside from the accuracy
of the curves, both for high light value and freedom from diffused
light in the field, and if the surfaces are allowed to become covered
with dust, his good work counts for little.
"If only the front surface needs cleaning, the method of cleaning
with cheese cloth, soap and water, as described above, answers very
well, but always throw away the first and, if necessary, the second
cloth, then wipe dry with a third or fourth cloth; but if the
surfaces all need cleaning I know of no better method than that of
taking the objective out of its cell, always using abundance of soap
and water, and keep in a good humor."
§ 69. The Preparation of Flat Surfaces of Rock Salt.
The preliminary grinding is accomplished as in the case of glass,
except that it goes on vastly faster. The polishing process is the
only part of the operation which presents any difficulty. The
following is an extract from a paper on the subject, by Mr. J. A.
Brashear, Pittsburg, Pa, U.S.A, from the Proceedings of the American
Association for the Advancement of Science, 1885. Practically the
same method was shown me by Mr. Cook some years earlier, so that I can
endorse all that Mr. Brashear says, with the following exceptions. We
consider that for small salt surfaces the pitch is better scored into
squares than provided with the holes recommended by Mr. Brashear.
Mr. Brashear's instructions are as follows. After alluding to the
difficulty of drying the polished salt surface--which is of course
wet--Mr. Brashear says:-
"Happily I have no trouble in this respect now, and as my method is
easily carried out by any physicist who desires to work with rock salt
surfaces, it gives me pleasure to explain it. For polishing a prism I
make an ordinary pitch bed of about two and one-half or three times
the area of the surface of the prism to be polished. While the pitch
is still warm I press upon it any approximately flat surface, such as
a piece of ordinary plate glass. The pitch bed is then cooled by a
stream of water, and conical holes are then drilled in the pitch with
an ordinary counter sink bit, say one-quarter of an inch in diameter,
and at intervals of half an inch over the entire surface. This is
done to relieve the atmospheric pressure in the final work. The upper
surface of the pitch is now very slightly warmed and a true plane
surface (usually a glass one, prepared by grinding and polishing three
surfaces in the ordinary way, previously wetted) is pressed upon it
until the pitch surface becomes an approximately true plane itself.
Fortunately, moderately hard pitch retains its figure quite
persistently through short periods and small changes of temperature,
and it always pays to spend a little time in the preparation of the
pitch bed.
"The polisher being now ready, a very small quantity of rouge and
water is taken upon a fine sponge and equally distributed over its
surface. The previously ground and fined salt surface (this work is
done the same as in glass working) is now placed upon the polisher and
motion instantly set up in diametral strokes. I usually walk around
the polisher while working a surface. It is well to note that motion
must be constant, for a moment's rest is fatal to good results, for
the reason that the surface is quickly eaten away, and irregularly so,
owing to the holes that are in the pitch bed. Now comes the most
important part of this method. After a few minutes' work the moisture
will begin to evaporate quite rapidly. No new application of water is
to be made, but a careful watch must be kept upon the pitch bed, and
as the last vestige of moisture disappears the prism is to be slipped
off the polisher in a perfectly horizontal direction, and if the work
has been well done, a clean, bright, and dry surface is the result.
The surface is now tested by the well-known method of interference
from a perfect glass test plate (see Fig. 178).
"If an error of concavity presents itself the process of polishing is
gone over again, using short diametral strokes. If the error is one
of convexity, the polishing strokes are to be made along the chords,
extending over the edge of the polisher. The one essential feature of
this method is the fact that the surface is wiped dry in the final
strokes, thus getting rid of the one great difficulty of pitch
polishing, a method undoubtedly far superior to that of polishing on
broadcloth. If in the final strokes the surface is not quite cleaned
I usually breathe upon the pitch bed, and thus by condensation place
enough moisture upon it to give a few more strokes, finishing just the
same as before. In ten minutes I have polished prisms of rock salt in
this manner that have not only shown the D line double, but Professor
Langley has informed me that his assistant, Mr. Keeler (J. E.), has
seen the nickel line clearly between the D lines. This speaks for the
superiority of the surfaces over those polished on broadcloth.
"In polishing prisms I prefer to work them on top of the polisher, as
they can be easily held, but as it is difficult to hold lenses or
planes in this way without injuring the surfaces, I usually support
them in a block of soft wood, turned so as to touch only at their
edges, and work the polisher over them. Though it takes considerable
practice to succeed at first, the results are so good that it well
repays the few hours' work it requires to master the few difficulties
it presents."
Fig. 58.
§ 70. Casting Specula for Mirrors.
According to Sir H. Grubb (loc. cit.) the best alloy is made of four
atoms of copper and one of tin; this gives by weight,
copper 252, tin 117.8.
The copper is melted first in a plumbago crucible; the tin is added
gradually. Of course, in the process of melting, even though a little
fine charcoal be sprinkled over the copper, some loss of that metal
will occur from oxidation. It is convenient in practice, therefore to
reserve a portion of the tin and test the contents of the crucible by
lifting a little of the alloy out and examining it.
The following indications may be noted: When the copper is in excess
the tint of the alloy is slightly red, and the structure, as shown at
a fractured surface, is coarsely crystalline. As the proper
proportions are more nearly attained, the crystalline structure
becomes finer, the colour whiter, and the crystals brighter. The
alloy is ready for use when the maximum brightness is attained and the
grain is fine.
If too much tin be added, the lustre diminishes. The correct
proportion is, therefore, attained when a further small addition of
tin produces no apparent increase of brightness or fineness of grain.
About three-quarters of the tin may be added at first, and the other
quarter added with testing as described. The alloy is allowed to cool
until on skimming the surface the metal appears bright and remains so
without losing its lustre by oxidation for a sensible time; it will
still be quite red-hot.
Fig. 59. Fig. 60.
As the speculum alloy is too difficult to work with ordinary tools, it
is best to cast the speculum of exactly the required shape and size.
This is done by means of a ring of iron turned inside (and out) and on
one edge. This ring is laid on a plate of figured iron, and before
the metal is poured the plate (G) (Figs 59 and 61) is heated to, say,
300° C. In order to avoid the presence of oxide as far as possible,
the following arrangements for pouring are made. A portion of the
lower surface of the ring is removed by radial filing until a notch
equal to, say, one-twentieth of the whole circumference is produced.
This is cut to an axial depth of, say, half an inch.
A bar of iron is then dovetailed loosely into the notch (Fig. 60, B),
so that it will rest on the iron plate, and half fill the notch. The
aperture thus left forms the port of ingress for the hot metal (see
Fig. 61, M). A bit of sheet iron is attached to the upper surface of
the ring, and lies as a sort of flap, shaped like a deep shovel,
against the outside of the ring overhanging the port (Figs. 59 and 61
at F). This flap does not quite reach the iron plate, and its sides
are bent so as to be in contact with the ring. A portion of a smaller
ring is then applied in such a manner as to form a pouring lip or pool
on the outside of the main ring at E, and the metal can only get into
the main ring by passing under the edge of the flap and up through the
port. This forms an efficient skimming arrangement. The process of
casting is carried out by pouring steadily into the lip.
To avoid air bubbles it is convenient to cause the metal to spread
slowly over the chill, and Mr. Nasmyth's method of accomplishing this
is shown in the figure (61). The chill rests on three pins, A B C
(Figs. 59 and 61). Before pouring begins the chill is tilted up off
C by means of the counterpoise D, which is insufficient to tilt it
after the speculum is poured. It is important that the chill should
be horizontal at the close of the operation, in order that the
speculum may be of even thickness throughout. This is noted by means
of levels placed on the ring (at K for instance).
Fig. 61.
This apparatus may appear unnecessarily complex, but it is worth while
to set it up, for it makes the operation of casting a speculum fairly
certain. If the metal is at the right temperature it will form a
uniformly liquid disc inside the ring. The mass sets almost directly,
and as soon as this occurs it is pushed to the edge of the plate and
the metal in the lip broken off by a smart upward tap with a hammer.
The dovetailed bit of iron is knocked downwards and falls off, and the
ring may then be lifted clear of the casting. The object of the
dovetail will now be understood, for without it there is great risk of
breaking into the speculum in knocking the "tail" off.
A box of quite dry sawdust is prepared in readiness for the process of
annealing before the speculum is cast. The box must be a sound wooden
or metal box, and must be approximately air-tight. For a speculum a
foot in diameter the box must measure at least 3 feet both ways in
plan, and be 2 feet 6 inches deep. Half the sawdust is in the box and
is well pressed down so as to half fill it. The other half must be
conveniently ready to hand. As soon as possible after casting, the
speculum is thrown into the box, covered over with the sawdust, and
the lid is put on.
The object in having the box nearly air-tight is to avoid
air-currents, which would increase the rate of cooling. A speculum a
foot in diameter may conveniently take about three days to anneal, and
should be sensibly warm when the box is opened on the fourth day. For
larger sizes longer times will be required. We will say that the
sawdust thickness on each side must be proportional to the dimensions
of the speculum, or may even increase faster with advantage if time is
of no moment.
The process of annealing may be considered successful if the disc does
not fly to pieces in working; it is to be worked on the chilled side.
The object of giving the chill the approximate counterpart form will
now appear; it saves some rough grinding, and causes the finished
surface to be more homogeneous than it would be if the centre were
sunk by grinding through the chilled surface.
In 1889 I learned from Mr. Schneider, Professor Row-land's assistant
at Baltimore, that in casting specula for concave gratings a good deal
of trouble had been saved by carrying out the operation in an
atmosphere consisting mostly of coal gas. It was claimed that in this
way the presence of specks of oxide was avoided. I did not see the
process in operation, but the results attained are known and admired
by all experimenters.
§ 71. Grinding and polishing Specula.
The rough grinding is accomplished by means of a lead tool and coarse
emery; the size of grain may be such as will pass a sieve of 60
threads to the inch. The process of grinding is quite similar to that
previously described, but it goes on comparatively quickly. The rough
grinding is checked by the spherometer, and is interrupted when that
instrument gives accordant and correct measurements all over the
surface.
The fine grinding may be proceeded with by means of a glass-faced tool
as before described, or the labour may be reduced in the following
manner. A slate tool, which must be free from green spots (a source
of uneven hardness), is prepared, and this is brought nearly to the
curvature of the roughly ground speculum, by turning or otherwise. It
is finished on the speculum itself with a little flour of emery. The
fine grinding is then carried on by means of slate dust and water, the
slate tool being the grinder. The tool is, of course, scored into
squares on the surface.
If the casting process has been carried out successfully, the rough
grinding may take, say six hours, and the fine grinding say thirty
hours for a disc a foot in diameter. The greatest source of trouble
is want of homogeneity in the casting, as evidenced by blowholes, etc.
In general, the shortest way is to discard the disc and start afresh
if there is any serious want of perfection in the continuity or
homogeneity of the metal.
Fig. 62.
The finely ground surface must, of course, be apparently correct in so
far as a spherometer (with 3 inches between the legs for a disc 1 foot
in diameter) will show. Polishing and figuring are carried out
simultaneously. Half an hour's polishing with a slate-backed pitch
tool and rouge and water will enable an optical test to be made. The
most convenient test is that of Foucault, a simple appliance for the
purpose being shown in the figure (62). It essentially consists of a
small lamp surrounded by an opaque chimney (A) through which a minute
aperture (pin-hole) is made. A small lens may be used, of very great
curvature, or even a transparent marble to throw an image of the flame
on the pin-hole.
A screen (B) is placed close to the source, and is provided with a
rocking or tilting motion (C) in its own plane. The source and screen
are partly independent, and each is provided with a fine adjustment
which serves to place it in position near the centre of curvature.
The screen is so close to the pin-hole in fact that both the source
and a point on the edge of the screen may be said to be at the centre
of curvature of the mirror. The mirror is temporarily mounted so as
to have its axis horizontal, in a cellar or other place of uniform
temperature.
The final focussing to the centre of curvature is made by the fine
adjustment screws; the image may be received on a bit of paper placed
on the screen and overlapping the edge nearest the source. The screws
are worked till the image has its smallest dimension and is bisected
by the edge of the screen. The test consists in observing the
appearance of the mirror surface while the screen is tilted to cut off
the light, as seen by an eye placed at the edge of the screen, a
peephole or eye lens being provided to facilitate placing the eye in a
correct position. The screen screws are worked so as to gradually cut
off the light, and the observer notes the appearance of the mirror
surface. If the curves are perfect and spherical, the transition from
complete illumination to darkness will be abrupt, and no part of the
mirror will remain illuminated after the rest.
For astronomical purposes a parabolic mirror is required. In this
case the disc may be partially screened by zonal screens, and the
position of the image for different zones noted; the correctness or
otherwise of the curvature may then be ascertained by calculation. A
shorter way is to place the source just outside the focus, to be found
by trial, and then, moving the extinction screen (now a separate
appliance) to, say, five times the radius of curvature away, where the
image should now appear, the suddenness of extinction may be
investigated. This, of course, involves a corresponding modification
of the apparatus.
Whether the tests indicate that a deepening of the Centre, i.e.
increase of the curvature, or a flattening of the edges is required,
at least two remedial processes are available. The "chisel and
mallet" method of altering the size of the pitch, squares of the
polisher may be employed, or paper or small pitch tools may be used to
deepen the centre. The "chisel and mallet" method merely consists in
removing pitch squares from a uniformly divided tool surface by means
of the instruments mentioned. This removal is effected at those
points at which the abrasion requires to be reduced.
When some practice is attained, I understand that it is usual to try
for a parabolic form at once, as soon as the polishing commences.
This is done by dividing the pitch surface by V-shaped grooves, the
sides of the grooves being radii of the circular surface, so that the
central parts of the mirror get most of the polishing action. If
paper tools are used they must not be allowed much overhang, or the
edges of the mirror betray the effects of paper elasticity. Most
operators "sink" the middle, but the late Mr. Lassell, a most
accomplished worker, always attained the parabolic form by reducing
the curvature of the edges of a spherical mirror.
§ 72. Preparation of Flat Surfaces.
As Sir H. Grubb has pointed out, this operation only differs from
those previously described in that an additional condition has to be
satisfied. This condition refers to the mean curvature, which must be
exact (in the case of flats it is of course zero) to a degree which is
quite unnecessary in the manufacture of mirrors or lenses.
A little consideration will show that to get a surface flat the most
straightforward method is to carry out the necessary and sufficient
condition for three surfaces to fit each other impartially. If they
each fit each other, they must clearly all be flat. To carry out the
process of producing a flat surface, therefore, two tools are made,
and the glass or speculum is ground first on one and then on the
other, the tools being kept "in fit" by occasional mutual grinding.
The grinding and polishing go on as usual. If paper is employed, care
must be taken that the polisher is about the same size as the object
to be polished.
There is a slight tendency to polish most at the edges; but if the
sweeps are of the right shape and size, this may be corrected
approximately. The best surfaces which have come under my notice are
those prepared as "test surfaces" by Mr. Brashear of Alleghany, Pa,
U.S.A. These I believe to be pitch polished. A pitch bed is
prepared, I presume, in a manner similar to that described for
rocksalt surfaces; but the working of the glass is an immense art, and
one which I believe--if one may judge by results--is only known to
Mr. Brashear.
In general, the effect of polishing will be to produce a convex or
concave surface, quite good enough for most purposes, but distinctly
faulty when tested by the interference fringes produced with the aid
of the test plate. The following information therefore--which I draw
from Mr. 'Cook--will not enable a student to emulate Mr. Brashear,
but will undoubtedly help him to get a very much better surface than
he usually buys at a high price, as exhibited on a spectroscope prism.
The only difference between this process and the one described for
polishing lenses, lies in the fact that the rouge is put into the
paper surface while the latter is wet with a dilute gum "mucilage."
It is of course assumed that the object and the two tools have been
finely ground and fit each other impartially. The paper is rubbed
over with rouge and weak gum water. The tool, when dry, is applied to
the flat ground surface (of the object), and is scraped with the
three-cornered file chisel as formerly described. This process must
be very carefully carried out. The paper must be of the quality
mentioned, or may even be thinner and harder. The cross strokes
should be more employed than in the case of the curved surfaces.
A good deal will depend on the method employed for supporting the
work; it is in general better to support the tool, which may have a
slate backing of any desired thickness, whereby the difficulty
resulting from strains is reduced. The work must be mounted in such a
way as to minimise the effect of changes of temperature. If a pitch
bed is selected, Mr. Brashear's instructions for rock salt may be
followed, with, of course, the obvious necessary modifications. See
also next section.
§ 73. Polishing Flat Surfaces on Glass or on Speculum Metal.
The above process may be employed for speculum metal, or pitch may be
used. In the latter case a fresh tool must be prepared every hour or
so, because the metal begins to strip and leave bits on the polisher;
this causes a certain amount of scratching to take place. As against
this disadvantage, the process of polishing, in so far as the state of
the surface is concerned, need not take an hour if the fine grinding
has been well done.
For the finest work changes of temperature, as in the case of glass,
cause a good deal of trouble, and the operator must try to arrange his
method of holding the object so as to give rise to the least possible
communication of heat from the hand.
The partial elasticity of paper, which is its defect as a polishing
backing, is, I believe, partly counterbalanced by the difficulty of
forming with pitch an exact counterpart tool without introducing a
serious rise of temperature (i.e. warming the pitch). The rate of
subsidence of the latter is very slow at temperatures where it is hard
enough to work reliably as a polisher.
A student interested in the matter of flat surfaces will do well to
read an account of Lord Rayleigh's work on the subject, Nature, vol.
xlviii, 1893, pp. 212, 526 (or B. A. Reports, 1893). In the first
of these communications Lord Rayleigh describes the method of using
test plates, and shows how to obtain the interference fringes in the
clearest manner.
For the ordinary optician a dark room and a soda flame afford all
requisite information; and if a person succeeds in making three glass
discs, say 6 inches in diameter, so flat that, when superposed in any
manner, the interference fringes are parallel and equidistant, even to
the roughest observation, he has nothing to learn from any book ever
written on glass polishing. Lord Rayleigh has also shown how to use
the free clean surface of water as a natural test plate.
Since the above was written the following details of his exact course
of procedure have been sent to me by Mr. Brashear, and I hereby tender
my thanks:-
"It really takes years to know just what to do when you reach that
point where another touch either gives you the most perfect results
attainable, or ruins the work you have already done. It has taken us
a long time to find out how to make a flat surface, and when we were
called upon to make the twenty-eight plane and parallel surfaces for
the investigation of the value of the metre of the international
standard, every one of which required an accuracy of one-twentieth of
a wave length, we had a difficult task to perform. However, it was
found that every surface had the desired accuracy, and some of them
went far beyond it.
It is an advantage in making flat surfaces to make more than one at a
time; it is better to make at least three, and in fact we always grind
and 'fine' three together. In making speculum plates we get up ten or
twelve at once on the lead lap. These speculum plates we can test as
we go on by means of our test plane until we get them nearly flat. In
polishing them we first make quite a hard polisher, forming it on a
large test plane that is very nearly correct. We then polish a while
on one surface and test it, then on a second and test it, and after a
while we accumulate plates that are slightly concave and slightly
convex. By working upon these alternately with the same polisher, we
finally get our polisher into such shape that it approximates more and
more to a flat surface, and with extreme care and slow procedure we
finally attain the results desired.
All our flats are polished on a machine which has but little virtue in
itself, unmixed with brains. Any machine giving a straight
diametrical stroke will answer the purpose. The glass should be
mounted so as to be perfectly free to move in every direction--that
is to say, perfectly unconstrained. We mount all our flats on a piece
of body Brussels carpet, so that every individual part of the woof
acts as a yielding spring. The flats are held in place by wooden
clamps at the edges, which never touch, but allow the bits of glass or
metal to move slowly around if they are circular; if they are
rectangular we allow them to tumble about as they please within the
frame holding them.
For making speculum metal plates either plane or concave we use
polishers so hard that they scratch the metal all over the surface
with fine microscopic scratches. We always work for figure, and when
we get a hard polisher that is in proper shape, we can do ever so many
surfaces with it if the environments of temperature are all right. If
we have fifty speculum flats to make, and we recently made three times
that number, we get them all ready and of accurate surface with the
hard polisher. Then we prepare a very soft polisher, easily indented
when cold with the thumb nail. A drop of rouge and about three drops
of water are put on the plate, and with the soft polisher about one
minute suffices to clean up all the scratches and leave a beautiful
black polish on the metal. This final touch is given by hand; if we
do not get the polish in a few minutes the surface is generally ruined
for shape, and we have to resort to the hard polisher again.
I assure you that nothing but patience and perseverance will master
the difficulty that one has to encounter, but with these two elements
'you are bound to get there.'"