atmospheric influence on the image, results from cutting down the
aperture of these mirrors by diaphragms, while the disadvantage
of reducing the separating power, is perceived at the same time.
Faint objects can be better seen with the whole surface than with a
reduced aperture, and this though apparently a property common to all
reflectors and object glasses is not so in reality. A defective edge
will often cause the whole field to be filled with a pale milky light,
which will extinguish the fainter stars. Good definition is just as
important for faint as for close objects.
The properties of these mirrors have been best shown by the excellence
of the photographs taken with them. Although these are not as sharp as
the image seen in the telescope, yet it must not be supposed that an
imperfect mirror will give just as good pictures. A photograph which
is magnified to 3 feet, represents a power of 380. As the original
negative taken at the focus of the mirror is not quite 1-1/2 inch in
diameter when the moon is at its mean distance, it has to be enlarged
about 25 times, and has therefore to be very sharp to bear it.
The light collecting power of an unsilvered mirror is quite surprising.
With a 15-1/2 inch, the companion of α Lyræ can be perceived, though it
is only of the eleventh magnitude. The moon and other bright objects
are seen with a purity highly pleasing to the eye, some parts being
even more visible than after silvering.
In order to finish this description, one part more of the optical
apparatus requires to be noticed--the plane mirrors. In the Newtonian
reflector the image is rejected out at the side of the tube by a flat
surface placed at 45° with the optical axis of the large concave.[7]
If this secondary mirror is either convex or concave, it modifies the
image injuriously, causing a star to look like a cross, and this though
the curvature be so slight as hardly to be perceptible by ordinary
means. For a long time I used a piece 3 × 5 inches, which was cut from
the centre of a large looking-glass accidentally broken, but eventually
found that by grinding three pieces of 6 inches in diameter against
one another, and polishing them on very hard pitch, a nearer approach
to a true plane could be made. They were tested by being put in the
telescope, and observing whether the focus was lengthened or shortened,
and also by trial on a star. When sufficiently good to bear these
tests, a piece of the right size was cut out with a diamond, from the
central parts.
[7] A right-angled prism cannot be used with advantage to replace the
plane silvered mirrors, because it transmits less light than they
reflect, is more liable to injure the image, and the glass is apt to
be more or less colored. Its great size and cost, one three inches
square on two faces being required for my purposes, has also to be
considered.
§2. THE TELESCOPE MOUNTING.
The telescope is mounted as an altitude and azimuth instrument, but
in a manner that causes it to differ from the usual instrument of
that kind. The essential feature is, that _the eye-piece or place of
the sensitive plate is stationary at all altitudes_, the observer
always looking straight forward, and never having to stoop or assume
inconvenient and constrained positions.
[Illustration: Fig. 27. Miss Herschel’s Telescope.]
The stationary eye-piece mounting was first used by Miss Caroline
Herschel, who had a 27 inch Newtonian arranged on that plan. Fig. 27.
(Smyth’s Celestial Cycle.)
Subsequently it was applied to a large telescope by Mr. Nasmyth, the
eminent engineer, but no details of his construction have reached
me. He used it for making drawings of the moon, which are said to be
excellently executed.
When it became necessary to determine how my telescope should be
mounted, I was strongly urged to make it an equatorial. But after
reflecting on the fact that it was intended for photography, and that
absolute freedom from tremor was essential, a condition not attained in
the equatorial when driven by a clock, and in addition that in the case
of the moon rotation upon a polar axis does not suffice to counteract
the motion in declination, I was led to adopt the other form.
A great many modifications of the original idea have been made. For
instance, instead of counterpoising the end of the tube containing
the mirror by extending the tube to a distance beyond the altitude or
horizontal axis, I introduced a system of counterpoise levers which
allows the telescope to work in a space little more than its own focal
length across. This construction permits both ends of the tube to be
supported, the lower one on a wire rope, and gives the greatest freedom
from tremor, the parts coming quickly to rest after a movement. In the
use of the telescope for photography, as we shall see, the system of
bringing the mass of the instrument to complete rest before exposing
the sensitive plate, and only driving that plate itself by a clock, is
always adopted.
The obvious disadvantage connected with the alt-azimuth mounting--the
difficulty of finding some objects--has not been a source of
embarrassment. In fact the instability of the optical axis in
reflecting instruments, if the mirror is unconstrainedly supported, as
it should be, renders them unsuitable for determinations of position. A
little patience will enable an observer to find all necessary tests, or
curious objects.
The mounting is divided into: a. The Tube; and b. The supporting frame.
a. _The Tube._
The telescope tube is a sixteen sided prism of walnut wood, 18 inches
in diameter, and 12 feet long. The staves are 3/8 of an inch thick,
and are hooped together with four bands of brass, capable of being
tightened by screws. Inside the tube are placed two rings of iron, half
an inch thick, reducing the internal diameter to about 16 inches. At
opposite sides of the upper end of the tube are screwed the perforated
trunnions _a_, Fig. 28 (of which only one is shown), upon which it
swings. Surrounding the other end is a wire rope _b b′ b″_, the ends of
which go over the pulleys _c_ (_c′_ not shown) on friction rollers, and
terminate in disks of lead _d d′_. These counterpoises are fastened on
the ends of levers _e e′_, which turn below on a fixed axle _f_.
By this arrangement as the tube assumes a horizontal position and
becomes, so to speak, heavier, the counterpoises do the same, while
when the tube becomes perpendicular, and most of its weight falls upon
the trunnions, the counterpoises are carried mostly by their axle. A
continual condition of equilibrium is thus reached, the tube being
easily raised or depressed to any altitude desired. It is necessary,
however, to constrain the wire rope _b b′ b″_, to move in the arc of
the circle described by the end of the tube and ends of the levers
and hence the twelve rollers or guide pulleys _g g′ g″_. Over some of
the same pulleys a thin wire rope _h h′_ runs, but while its ends are
fastened to the lower part of the tube at _b_, the central parts go
twice around a roller connected with the winch _i_, near the eye-piece,
thus enabling the observer to move the telescope in altitude, without
taking the eye from the eye-piece.
[Illustration: Fig. 28. Sectional View of Observatory.]
The iron wire rope required to be carefully made, so as to avoid
rigidity. It contains 2-1/3 miles of wire, 1/100 of an inch in
diameter, and has 300 strands. Each single wire will support 7 pounds.
It is, however, more flexible than a hempen rope of the same size,
owing to its loose twisting.
[Illustration: Fig. 29. The Mirror Support.]
At the lower end of the tube, at the distance of a foot, and crossing
it at right angles, held by three bars of iron _i i′ i″_, Fig. 29, is a
circular table of oak _e_, which carries an India-rubber air sac _d_,
and upon this the mirror _f_ is placed. The edge support of the mirror
is furnished by a semicircular band of tin-plate _a_, lined inside with
cotton, and fastened at the ends by links of chain _b_, (_b′_ not seen)
to two screws _c c′_; _g_ and _h_ are the wire ropes, marked _b_ and
_h_ in Fig. 28.
Instead of the blanket support which Herschel found so advantageous,
M. Foucault has suggested this use of an air sac. In his instrument
there is a tube going up to the observer, by which he may adjust its
degree of inflation. It requires that there should be three bearings
_c c′ c″_, in front of the mirror, against which it may press when
the sac behind is inflated, otherwise the optical axis is altogether
too instable, and objects cannot be found. The arrangement certainly
gives beautiful definition, bringing stars to a disk when the glass
just floats, without touching its front bearings. The first sac that I
made was composed of two circular sheets of India-rubber cloth, joined
around the edges. But this could not be used while photographing,
because the image was kept in a state of continuous oscillation if
there was a breeze, and even under more favorable circumstances took a
long time to come to rest. It was not advisable to blow the mirror hard
up against its three front bearings, in order to avoid the instability,
for then every point in of an object became triple. To the eye the
oscillations were not offensive, because the swaying image was sharp.
Subsequently, however, an air chair cushion was procured, and as the
surface was flat instead of convex the difficulty became so much less,
that the blanket support was definitely abandoned. It is necessary that
the mirror should have free play in the direction of the length of the
tube when this kind of support is used, and that is the reason why the
tin edge hoop must terminate in links of chain.
The interval, eight or ten inches, which separates the face of the
mirror from the tube, is occupied by a curtain of black velvet,
confined below by a drawing cord and tacked above to the tube. This
permits access to the mirror to put a glass cover on it, and when shut
down stops the current of air rushing up. When the instrument is not
being used this curtain is left open, because the mirror and tube are
in that case kept more uniform in temperature with the surrounding air.
In spite of such contrivances there is still sometimes a strong
residual current in the tube. I have tried to overcome it by covering
the mouth of the tube with a sheet of flat glass, but have been obliged
to abandon that because the images were injured. At one time, too, when
it was supposed that the current was partly from the observer’s body,
heated streams of air going out around the tube, the aperture in the
dome was closed by a conical bag of muslin, which fitted the mouth of
the telescope tightly. The only advantages resulting were mere bodily
comfort and a capability of perceiving fainter objects than before,
because the sky-light was shut off.
[Illustration: Fig. 30. Section of Azimuth Axis.]
b. _The Supporting Frame._
The frame which carries the preceding parts is of wood, and rests on
a vertical axis _a_, Fig. 30, turning below in a gun-metal cup _b_,
supported by a marble block resting on the solid rock. The upper end of
the axis is sustained by two collars, one _c c′_ above, and the other
below an intermediate triangular box _e e′_ from the sides of which
three long beams _f f f_ 12 × 3 inches diverge, gradually declining
till they meet the solid rock at the limits of the excavation in
which the observatory is placed. These beams are fastened together
by cross-pieces _g g g_, Fig. 31, and go through the floor in spaces
_h h h_, so contrived that the floor does not touch them. At the ends
they are cased with a thick leaden sheathing, to deaden vibration and
prevent the access of moisture.
[Illustration: Fig. 31. Plan of Observatory (lower floor).]
This tripod support in connection with the sustaining of the telescope
by the wire rope, gives that steadiness which is so essential in
photography. Only a slight amount of force, about two pounds, is
required to move the instrument in azimuth, though it weighs almost a
thousand pounds.
The plan of the frame centrally carried by the axis _a_ is as follows:
From the corners of a parallelogram _i i_ (2 × 13 feet) of wooden
beams, eight inches thick and three inches broad, perpendiculars _n
n′_, Fig. 28, rise. At the top they are connected by lighter pieces to
form a parallelogram, similar to that below, and just large enough to
contain the tube of the telescope. At right angles to the parallelogram
below, and close upon it, a braced bar _o o′_, Fig. 28, crosses.
From its extremities four slanting braces as at _p p′_, Fig. 28, go
to the corners of the upper parallelogram, and combine to give it
lateral support. At the top of one close pair of the perpendiculars
_n′_, Fig. 28, are bronze frames carrying friction rollers upon which
the trunnions move, while similarly upon the other pair _n_ are two
pulleys, also on friction rollers, for the wire rope coming from the
counterpoises.
Movement in altitude is very easily accomplished, and with the left
hand upon the winch _i_, under high powers, both altitude and azimuth
motions are controlled, and the right hand left free. The whole
apparatus works so well, that in ordinary observation the want of a
clock movement has not been felt. Of course for photography that is
essential.
§3. THE CLOCK MOVEMENT.
The apparatus for following celestial bodies is divided into two parts;
a. The Sliding Plate-holder; and b. The Clepsydra. In addition a short
description of the Sun-Camera, c, is necessary.
a. _The Sliding Plate-holder._
Mr. De La Rue, who has done so much for celestial photography, was the
first to suggest photographing the moon on a sensitive plate, carried
by a frame moving in the apparent direction of her path. He never,
however, applied an automatic driving mechanism, but was eventually
led to use a clock which caused the whole telescope to revolve upon a
polar axis, and thus compensate for the rotation of the earth, and on
certain occasions for the motion of the moon herself. In this way he
has produced the best results that have been obtained in Europe. Lord
Rosse, too, employed a similar sliding plate-holder, but provided with
clock-work to move it at an appropriate rate. I have not been able as
yet to procure any precise account of either of these instruments.
The first photographic representations of the moon ever made, were
taken by my father, Professor John W. Draper, and a notice of them
published in his quarto work “On the Forces that Organize Plants,”
and also in the September number, 1840, of the London, Edinburgh, and
Dublin Philosophical Magazine. He presented the specimens to the New
York Lyceum of Natural History. The Secretary of that Association has
sent me the following extract from their minutes:--
“_March 23d, 1840._ Dr. Draper announced that he had succeeded
in getting a representation of the moon’s surface by the
Daguerreotype.... The time occupied was 20 minutes, and the size of
the figure about 1 inch in diameter. Daguerre had attempted the same
thing, but did not succeed. This is the first time that anything like
a distinct representation of the moon’s surface has been obtained.
“ROBT. H. BROWNNE, _Secretary_.”
As my father was at that time however much occupied with experiments
on the Chemical Action of Light, the Influence of Light on the
Decomposition of Carbonic Acid by Plants, the Fixed Lines of the
Spectrum, Spectrum Analysis, &c., the results of which are to be found
scattered through the Philosophical Magazine, Silliman’s Journal, and
the Journal of the Franklin Institute, he never pursued this very
promising subject. Some of the pictures were taken with a three inch,
and some with a five inch lens, driven by a heliostat.
In 1850, Mr. Bond, taking advantage of the refractor of 15 inches
aperture at Cambridge, obtained some fine pictures of the moon, and
subsequently of double stars, more particularly Mizar in Ursa Major.
The driving power, in this instance, was also applied to move the
telescope upon a polar axis.
Besides these, several English and continental observers, Messrs.
Hartnup, Phillips, Crookes, Father Secchi, and others, have worked at
this branch of astronomy, and, since 1857, Mr. Lewis M. Rutherfurd, of
New York, has taken many exquisite lunar photographs, which compare
favorably with foreign ones.
But in none of these instances has the use of the sliding plate-holder
been persisted in, and its advantages brought into view. In the first
place it gets rid completely of the difficulties arising from the
moon’s motion in declination, and in the second, instead of injuring
the photograph by the tremors produced in moving the whole heavy mass
of a telescope weighing a ton or more, it only necessitates the driving
of an arrangement weighing scarcely an ounce.
My first trials were with a frame to contain the sensitive plate, held
only at three points. Two of these were at the ends of screws to be
turned by the hands, and the third was on a spring so as to maintain
firm contact. This apparatus worked well in many respects, but it was
found that however much care might be taken, the hands always caused
some tremor in the instrument. It was evident then that the difficulty
from friction which besets the movements of all such delicate
machinery, and causes jerking and starts, would have to be avoided in
some other way.
I next constructed a metal slide to run between two parallel strips,
and ground it into position with the greatest care. This, when set in
the direction of the moon’s apparent path, and moved by one screw,
worked better than the preceding. But it was soon perceived that
although the strips fitted the frame as tightly as practicable, an
adhesion of the slide took place first to one strip and then to the
other, and a sort of undulatory or vermicular progression resulted.
The amount of deviation from a rectilinear motion, though small, was
enough to injure the photographs. At this stage of the investigation
the regiment of volunteers to which I belonged was called into active
service, and I spent several months in Virginia.
My brother, Mr. Daniel Draper, to whose mechanical ingenuity I have
on several occasions been indebted for assistance in the manifold
difficulties that have arisen while constructing this telescope,
continued these experiments at intervals. He presented me on my return
with a slide and sand-clock, with which some excellent photographs have
been taken. He had found that unless the slide above mentioned was made
ungovernably long, the same trouble continued. He then ceased catching
the sliding frame _h_, Fig. 32, by two opposite sides, and made it run
along a single steel rod _a_, being attached by means of two perforated
plates of brass _b_, _b′_. The cord _i_ going to the sand-clock, was
applied so as to pull as nearly as possible in the direction of the
rod. A piece of cork _c_, gave the whole steadiness, and yet softness
of motion. The lower end of the frame was prevented from swinging back
and forward by a steel pin _d_, which played along the glass rod _e_.
All these parts were attached to a frame _k_, fitting on the eyepiece
holder, and permitting the rod _a_ to change from the horizontal
position in which it is here drawn, to any angular one desired. The
thumb-screw _f_ retained it in place; _g_ and _g′_ are pulleys which
permit the cord to change direction.
[Illustration: Fig. 32. Sliding Plate-holder.]
Subsequently, a better method of examining the uniformity of the
rate, than by noticing the sharpness of the photograph produced, was
invented. It consists in arranging a fixed microscope, magnifying
about 40 times, at the back of the ground glass plate, which fits in
the same slide as the sensitive plate. By watching the granulated
appearance pass before the eye, as the slide is moved by the clock, the
slightest variation from uniformity, any pulsatile or jerking movement
is rendered visible. By the aid of this microscopic exaggeration, it
was seen that occasionally, when there had been considerable changes in
temperature, the steadiness of the motion varied. This was traced to
the irregular slipping of _b_, _b′_.
[Illustration: Fig. 33. Frictionless Slide (front view).
Sectional view.]
A different arrangement was then adopted, by which a lunar crater can
be kept bisected as long as is necessary, and which gives origin to no
irregularities, but pursues a steady course. The principle is, not to
allow a slipping friction anywhere, but to substitute rolling friction,
upon wheels turning on points at the ends of their axles. The following
wood-cut is half the real size of this arrangement.
A glass rod _a_, _a′_, Fig. 33, is sustained by two wheels _b_, _b′_,
and kept in contact, with them by a third friction roller _c_, pressed
downward by a spring. This rod carries a circular frame _d_, _d′_, upon
which at _e_, _e′_, _e″_, are three glass holders and platinum catches.
A spring _f_ holds the sensitive plate in position, by pressing
against its back. The circular frame _d_ is kept in one plane by a
fourth friction roller _g_, which runs on a glass rod _h_, and is kept
against it by the inward pressure of the overhanging frame _d_. The
cord _i_ is attached to the arm _k_, and pulls in the direction of the
glass rod _a_. From _m_ to a fixed point near _b_, a strip of elastic
India-rubber is stretched, to keep the cord tight. The ring of brass
_n_, _n′_ carries the whole, serving as a basis for the stationary
parts, and in its turn being fastened to the eyepiece holder, so as
to allow the glass rod _a_ to change direction, and be brought into
coincidence with the apparent path of the moon. At _o_ is a thumb-screw
or clamp. Through the ring _n_, _n′_, a groove _p_ is cut, into which
a piece of yellow glass may be placed, when the actinic rays are to be
shut off from the plate.
Since this contrivance has been completed, all the previous
difficulties have vanished. The moving of a plate can be accomplished
with such precision, that when the atmosphere was steady, negatives
were taken which have been enlarged to three feet in diameter.
The length of time that such a slide can be made to run is indefinite,
depending in my case on the size of the diagonal flat mirror, and
aperture of the eyepiece holder. I can follow the moon for nearly four
minutes, but have never required to do so for more than fifty seconds.
At the mouth of the instrument, where no secondary mirror is necessary,
the time of running could be increased.
The setting of the frictionless slide in angular position is
accomplished as follows: A ground glass plate is put into it, with
the ground face toward the mirror. Upon this face a black line must
have been traced, precisely parallel to the rod _a_. This may be
accomplished by firmly fixing a pencil point against the ground side,
and then drawing the frame d and glass past it, while the rest of the
slide is held fast. As the moon passes across the field, the position
of the apparatus must be changed, until one of the craters runs along
the line from end to end. A cross line drawn perpendicular to the
other, serves to adjust the rate of the clepsydra as we shall see, and
when a crater is kept steadily on the intersection for twice or three
times the time demanded to secure an impression, the adjustment may be
regarded as complete.
It is necessary of course to expose the sensitive plate soon after,
or the apparent path of the moon will have changed direction, unless
indeed the slide is set to suit a future moment.
b. _The Clepsydra._