building, and running into a tank _i_, Fig. 38, which will contain
a ton weight. The roof exposes a surface of 532 square feet, and
consequently a fall of rain equal to one inch in depth, completely
fills the tank. During the course of the year the fall at this place
is about 32 inches, so that there is always an abundance. In order
to keep the water free from contamination, the roof is painted with
a ground mineral compound, which hardens to a stony consistence, and
resists atmospheric influences well. The tank is lined with lead, but
having been in use for many years for other purposes, is thoroughly
coated inside with various salts of lead, sulphates, &c. In addition
the precaution is taken of emptying the tank by a large stopcock when a
rainstorm is approaching, so that any accumulation of organic matter,
which can reduce nitrate of silver, may be avoided. It has not been
found feasible to use the well or spring water of the vicinity.
The tank is placed close under the eaves of the building, so as to gain
as much head of water as is desirable. From near its bottom a pipe
terminating in a stopcock _k_, Fig. 38, passes into the Laboratory.
In the northeast corner of the room, and under the tap is a sink for
refuse water and solutions, and over which the negatives are developed.
It is on an average about twelve feet distant from the telescope. In
another corner of the room is a stove, resembling in construction an
open fireplace, but sufficient nevertheless to raise the temperature to
80° F. or higher, if necessary. As a provision against heat in summer,
the walls and roof are double, and a free space with numerous openings
above is left for circulation of air, drawn from the foundations.
The roof is of tinplate, fastened directly to the rafters, without
sheathing, in order that heat may not accumulate to such an extent
during the day as to constitute a source of disturbance when looking
across it at night.
For containing negatives, which from being unvarnished require
particular care, there is at one side of the room a case with twenty
shallow drawers each to hold eighteen. They accumulate very rapidly,
and were it not for frequent reselections the case would soon be
filled. On some nights as many as seventeen negatives have been taken,
most of which were worthy of preservation. Not less than 1500 were made
in 1862 and ’63.
b. _Photographic Processes._
In photographic manipulations I have had the advantage of my father’s
long continued experience. He worked for many years with bromide and
chloride of silver in his photo-chemical researches (Journal of the
Franklin Institute, 1837), and when Daguerre’s beautiful process was
published, was the first to apply it to the taking of portraits (Phil.
Mag., June, 1840) in 1839; the most important of all the applications
of the art. Subsequently he made photographs of the interference
spectrum, and ascertained the existence of great groups of lines _M_,
_N_, _O_, _P_, above _H_, and totally invisible to the naked eye (Phil.
Mag., May, 1843). The importance of these results, and of the study of
the structure of flames containing various elementary bodies, that he
made at the same time, are only now exciting the interest they deserve.
In 1850, when his work on Physiology was in preparation, and the
numerous illustrations had to be produced, I learnt microscopic
photography, and soon after prepared the materials for the collodion
process, then recently invented by Scott Archer. We produced in 1856
many photographs under a power of 700 diameters, by the means described
in the next section.
At first the usual processes for portrait photography were applied to
taking the Moon. But it was soon found necessary to abandon these and
adopt others. When a collodion negative has to be enlarged--and this
is always the case in lunar photography, where the original picture
is taken at the focus of an object glass or mirror--imperfections
invisible to the naked eye assume an importance which causes the
rejection of many otherwise excellent pictures. Some of these
imperfections are pinholes, coarseness of granulation in the reduced
silver, liability to stains and markings, spots produced by dust.
These were all avoided by washing off the free nitrate of silver
from the sensitive plate, before exposing it to the light, and again
submitting it to the action of water, and dipping it back into the
nitrate of silver bath before developing. The quantity of nitrate
of silver necessary to development when pyrogallic acid is used, is
however better procured by mixing a small quantity of a standard
solution of that salt with the acid.
The operation of taking a lunar negative is as follows. The glass
plates 2-3/4 × 3-1/4 inches are kept in nitric acid and water until
wanted. They are then washed under a tap, being well rubbed with
the fingers, which have of course been properly cleaned. They are
wiped with a towel kept for the purpose. Next a few drops of iodized
collodion are poured on each side, and spread with a piece of cotton
flannel. They are then polished with a large piece of this flannel,
and deposited in a close dry plate box. This system of cleaning with
collodion was suggested by Major Russel, to whose skilful experiments
photography is indebted for the tannin process. It certainly is most
effective, the drying pyroxyline removing every injurious impurity.
There is never any trouble from dirty plates.
The stock of plates for the night’s work, a dozen or so, being thus
prepared, one of them is taken, and by movement through the air is
freed from fibres of cotton. It is then coated with filtered collodion
being held near the damp sink. The coated plate, when sufficiently
dry, is immersed in a 40 grain nitrate of silver bath, acidified with
nitric acid until it reddens litmus paper. The exact amount of acid in
the bath makes in this “Washed Plate Process” but little difference.
When the iodide and bromide of silver are thoroughly formed the plate
is removed, drained for a moment, and then held under the tap till all
greasiness, as it is called, disappears. Both front and back receive
the current in turn.
It is then exposed, being carried on a little wooden stand, Fig. 43,
covered with filtering paper to the telescope, and deposited on the
sliding plateholder which has been set to the direction and rate of
the moon, while the plate was in the bath. The time of exposure is
ascertained by counting the beats of a half-second pendulum.
The method by which exposure without causing tremor is accomplished, is
as follows: A yellow glass slides through the eyepiece-holder, Fig. 33,
just in front of the sensitive plate, and is put in before the plate.
The yellow-colored moon is centred on the collodion film, and the
clepsydra and slide are set in motion, the mass of the telescope being
at rest. A pasteboard screen is put in front of the telescope, and the
yellow glass taken out. After 20 seconds the instrument remaining still
untouched and motionless, the screen is withdrawn, and as many seconds
allowed to elapse as desirable. The screen is then replaced and the
plate taken back to the photographic room.
[Illustration: Fig. 43. Plate Carrier.]
After being again put under the tap to remove any dust or impurity,
it is dipped into the nitrate bath for a few seconds. Two drachms
of a solution of protosulphate of iron 20 grains, acetic acid 1
drachm, and water 1 ounce, is poured on it. As soon as the image is
fairly visible this is washed off, and the development continued if
necessary with a weak solution of pyrogallic acid and citro-nitrate of
silver--pyrogallic and citric acids each 1/5 grain, nitrate of silver
1/10 grain, water 1 drachm. In order to measure these small quantities
standard solutions of the substances are made, so that two drops of
each contain the desired amount. They are kept in bottles, through the
corks of which pipettes descend to just below the level of the liquid.
This avoids all necessity of filtering, and yet no blemishes are
produced by particles of floating matter.
[Illustration: Fig. 44. Pipette Bottle.]
During the earlier part of the development, when the protosulphate
of iron is on the film, an accurate judgment can be formed as to the
proper length of time for the exposure in the telescope. If the image
appears in 10 seconds, it will acquire an appropriate density for
enlargement in 45 seconds, and will have the minimum of what is called
fogging and the smallest granulations. If it takes longer to make its
first appearance the exposure must be lengthened, and vice versa.
[Illustration: Fig. 45. Developing Stand.]
The latter part of the development, when re-development is practised,
is purposely made slow, so that the gradation of tones may be varied
by changing the proportion of the ingredients. As it would be tiresome
and uncleanly to hold the plates in the hand, a simple stand is used
to keep them level. It consists of a piece of thin wood _a_, Fig. 45,
with an ordinary wood screw, as at _b_, going through each corner. Four
wooden pegs, as at _c_, furnish a support for the plate _d_. By the aid
of this contrivance and the washing system, I seldom get my fingers
marked, and what is much more important, rarely stain a picture.
When the degree of intensity most suitable for subsequent enlargement
is reached, that is, when the picture is like an overdone positive, the
plate is again flooded with water, treated with cyanide of potassium
or hyposulphite of soda, once more washed and set upon an angle on
filtering paper to dry. It is next morning labelled, and put away
unvarnished in the case.
To the remark that this process implies a great deal of extra trouble,
it can only be replied that more negatives can be taken on each night
than can be kept, and that, even were it not so, one good picture is
worth more than any number of bad ones.
Although the above is the method at present adopted, and by which
excellent results have been obtained, it may at any moment give place
to some other, and is indeed being continually modified. The defects
it presents are two--first, the time of exposure is too long, and
second, there is a certain amount of lateral diffusion in the thickness
of the film, and in consequence a degree of sharpness inferior to that
of the image produced by the parabolic mirror. The shortest time in
which the moon has been taken in this observatory has been one-third
of a second, on the twenty-first day, but on that occasion the sky
was singularly clear, and the intrinsic splendor of the light great.
The full moon under the same circumstances would have required a much
shorter exposure. A person, however, who has put his eye at the focus
of such a silvered mirror will not be surprised at the shortness of the
time needed for impressing the bromo-iodide film; the brilliancy is so
great that it impairs vision, and for a long time the exposed eye fails
to distinguish any moderately illuminated object. The light from 188
square inches of an almost total reflecting surface is condensed upon 2
square inches of sensitive plate.
Occasionally a condition of the sky, the reverse of that mentioned
above, occurs. The moon assumes a pale yellow color, and will
continue to be of that non-actinic tint for a month or six weeks.
This phenomenon is not confined to special localities, but may extend
over great tracts of country. In August, 1862, when our regiment was
encamped in Virginia, at Harper’s Ferry, the atmosphere was in this
condition there, and was also similarly affected at the observatory,
more than 200 miles distant. As to the cause, it was not forest or
prairie fires, for none of them of sufficient magnitude and duration
occurred, but was probably dust in a state of minute division. No
continued rain fell for several weeks, and the clay of the Virginia
roads was turned into a fine powder for a depth of many inches. The
Upper Potomac river was so low that it could be crossed dry-shod. On
a subsequent occasion when the same state of things occurred again,
I exposed a series of plates (whose sensitiveness was not less than
usual, as was proved by a standard artificial flame) to the image of
the full moon in the 15-1/2 inch reflector for 20 seconds, and yet
obtained only a moderately intense picture. This was 40 times as long
as common.
Upon all photographic pictures of celestial objects the influence of
the atmosphere is seen, being sometimes greater and sometimes less.
To obtain the best impressions, just as steady a night is necessary
as for critical observations. If the image of Jupiter is allowed to
pass across a sensitive plate, a streak almost as wide as the planet
is left. It is easily seen not to be continuous, as it would have
been were there no atmospheric disturbances, but composed of a set
of partially isolated images. Besides this planet, I have also taken
impressions of Venus, Mars, double stars, &c.
An attempt has been made to overcome lateral diffusion in the thickness
of the film by the use of dry collodion plates, more particularly
those of Major Russel and Dr. Hill Norris. These present, it is true,
a fine and very thin film during exposure, but while developing are
so changed by wetting in their mechanical condition that no advantage
has resulted. It was while trying them, that I ascertained the great
control that hot water exercises over the rapidity of development, and
time of exposure, owing partly no doubt to increase of permeability
in the collodion film, but also partly to the fact that chemical
decompositions go on more rapidly at higher temperatures. I have
attempted in vain to develop a tannin plate when it and the solutions
used were at 32° F., and this though it had had a hundred times the
exposure to light that was demanded when the plate was kept at 140° F.
by warm water.
Protochloride of palladium, which I introduced in 1859, is frequently
employed when it is desired to increase the intensity of a negative
without altering its thickness. This substance will augment the opacity
16 times, without any tendency to injure the image or produce markings.
It is only at present kept out of general use by the scarcity of the
metal.
§6. THE PHOTOGRAPHIC ENLARGER.
Two distinct arrangements are used for enlarging, _a_, for Low Powers
varying from 1 to 25; and _b_, for High Powers from 50 to 700 diameters.
a. _Low Powers._
The essential feature in this contrivance is an entire novelty in
photographic enlargement, and it is so superior to solar cameras, as
they are called, that they are never used in the observatory now. It
consists in employing instead of an achromatic combination of lenses, a
_mirror_ of appropriate curvature to magnify the original negatives or
objects. The advantages are easily enumerated, perfect coincidence of
visual and chemical foci, flat field, absolute sharpness of definition.
If the negative is a fine one, the enlarged proofs will be as good as
possible.
[Illustration: Fig. 46. The Photographic Enlarger.]
The mirror is of 9 inches aperture, and 11-1/2 inches focal length. It
was polished on my machine to an elliptical figure of 8 feet distance
between the conjugate foci, and was intended to magnify 7 times. At
first the whole mirror was allowed to officiate, the object being
illuminated by diffused daylight. But it was soon apparent, that
although a minute object placed in one focus was perfectly reproduced
at the other, seven times as large, yet a large one was not equally
well defined in all its parts.
I determined then to produce the enlarged image by passing a solar-beam
1-1/2 inch in diameter through the original lunar negative--placed in
the focus nearest to the mirror--and allowing it to fall on a portion
of the concave mirror, 1-1/2 inch in diameter, at one side of the
vertex. Being reflected, it returns past the negative, and goes to form
the magnified image at the other focus of the ellipse.
In Fig. 46, _a_ is the heliostat on a stone shelf outside; _b_ a
silvered glass mirror, to direct the parallel rays through _c_, the
negative; _d_ is the elliptical mirror; _e_ an aperture to be partly
closed by diaphragms; _f_ a rackwork movement carried by the tripod
_g_; the curtain _h h′_ shuts out stray light from the interior of the
observatory. The aperture _i_ is also diaphragmed, but is shown open to
indicate the position of the heliostat, the shelf of which joins the
outside of the building at _l_. The dotted line points out the course
of the light, which coming from the sun falls on the heliostat mirror
_a_, then on _b_, through _c_ to _d_, and thence returning through _e_
to the sensitive plate in the plate holder _k_.
The distance of this last can be made to vary, being either two feet
or twenty-eight feet from _d_. In the latter case a magnifying power
of about 25 results, the moon being made three feet in diameter. The
sensitive plate is carried by a frame, which screws to the side wall of
the building, and can be easily changed in position. The focussing is
accomplished by the rack _f_. Where so small a part (1-1/2 inch) of the
surface of the mirror is used, a rigid adherence then to the true foci
of this ellipse is not demanded, the mirror seeming to perform equally
well whether magnifying 7 or 25 times. Theoretically it would seem to
be limited to the former power.
If instead of placing a lunar photograph, which in the nature of the
case is never absolutely sharp, at _c_, some natural object, as for
instance a section of bone, is attached to the frame moved by _f_, then
under a power of 25 times it is as well defined as in any microscope,
while at the same time the amount of its surface seen at once is
much larger than in such instruments, and the field is flat. If the
intention were, however, to make microscopic photographs, a mirror of
much shorter focal length would be desirable, one approaching more to
those of Amici’s microscopes.
By the aid of a concave mirror used thus obliquely, or excentrically,
all the difficulties in the way of enlarging disappear, and pictures of
the greatest size can be produced in perfection. I should long ago have
made lunar photographs of more than 3 feet in diameter, except for the
difficulties of manipulating such large surfaces.
In order to secure a constant beam of sunlight a heliostat is placed
outside the observatory, at its southeast corner _f_, Fig. 38. This
beam, which can be sent for an entire day in the direction of the
earth’s axis, is intercepted as shown at _b_, Fig. 46, and thus if
needed an exposure of many hours could be given. The interior of the
observatory and photographic room being only illuminated by faint
yellow rays, no camera box is required to cut off stray light. The
eye is by these means kept in a most sensitive condition, and the
focussing can be effected with the critical accuracy that the optical
arrangement allows, no correction for chromatic aberration being
demanded.
I have made all the parts of this apparatus so that they can be easily
separated or changed. The flat mirrors are of silvered glass, and are
used with the silvered side toward the light, to avoid the double image
produced when reflection from both sides of a parallel plate of glass
is permitted. The large concave mirror happens to be of speculum metal,
but it can be repolished if necessary by means of a four inch polisher,
passed in succession over every chord of the face. A yellow film of
tarnish easily accumulates on metal specula if they are not carefully
kept, and decreases their photographic power seriously.
_Of the making of Reverses._--In addition to the use of the Enlarger
for magnifying, it is found to have important advantages in copying by
contact. The picture of the image of the moon produced in the telescope
is negative, that is, the lights and shades are reversed. In enlarging
such a negative reversal again takes place, and a positive results.
This positive cannot, however, be used to make prints on paper, because
in that operation reversing of light and shade once more occurs. It
is necessary then at some stage to introduce still another reversal.
This may be accomplished either by printing from the original negative
a positive, which may be enlarged, or else printing from the enlarged
positive a negative to make the paper proofs from. In either case a
collodion film, properly sensitized, is placed behind the positive or
negative, and the two exposed to light.
If diffused light or lamplight is used, the two plates must be as
closely in contact as possible, or the sharpness of the resulting
proof is greatly less than the original. This is because the light
finds its way through in many various directions. If the two plates,
however, are placed in the cone of sunlight coming from the Enlarger,
and at a distance of fifteen or twenty feet from it, the light passes
in straight lines and only in one direction through the front picture
to the sensitive plate behind. I have not been able to see under these
circumstances any perceptible diminution in sharpness, though the
plates had been 1/16 of an inch apart. It is perfectly feasible to use
wet collodion instead of dry plates, no risk of scratching by contact
is incurred, and the whole operation is easily and quickly performed.
The time of exposure, 5 seconds, is of convenient length, but may be
increased by putting a less reflecting surface or an unsilvered glass
mirror in the heliostat. A diaphragm with an aperture of half an inch
if placed at _e_, Fig. 46, to shut out needless light, and avoid
injuring the sharpness of the reverse by diffusion through the room.
In enlarging other diaphragms are also for the same reason put in the
place of this one. For a half moon for instance, a yellow paper with
a half circular aperture, whose size may be found by trial in a few
minutes, is pinned against _e_.
The enlarged pictures obtained by this apparatus are much better than
can be obtained by any other method known at present. The effect,
for instance, of a portrait, made life-size, is very striking. Some
astronomers have supposed that advantages would arise from taking
original lunar negatives of larger size in the telescope, that is,
from enlarging the image two or three times by a suitable eyepiece or
concave achromatic, before it reached the sensitive plate. But apart
from the fact that a reflector would then have all the disadvantages
of an achromatic, the atmospheric difficulties, which in reality
constitute the great obstacle to success, would not be diminished by
such means. The apparent advantage, that of not magnifying defects
in the collodion, is not of much moment, for when development of the
photographs is properly conducted, and thorough cleanliness practised,
imperfections are not produced, and the size of the silver granules is
not objectionable.
b. _High Powers._
Although negatives of astronomical objects have not as yet been made
which could stand the high powers of the arrangement about to be
described, yet they bear the lower powers well, and give promise of
improvement in the future.
Photography of microscopic objects as usually described, consists
in passing a beam of light through the transparent object into the
compound body of the microscope, and receiving it on its exit from the
eyepiece upon a ground glass or sensitive plate. The difficulty which
besets the instrument generally, and interferes with the production of
fine results, arises from the uncertainty of ascertaining the focus
or place for the sensitive plate. For if the collodion film be put
where the image on ground glass seems best defined, the resulting
photograph will not be sharp, because the actinic rays do not form
their image there, but either farther from or nearer to the lenses,
depending on the amount of the chromatic correction given by the
optician. Practically by repeated trials and variation of the place of
the sensitive compound, an approximation to the focus of the rays of
maximum photographic intensity is reached.
[Illustration: Fig. 47. Microscope for Photography.]