Probably the most convenient and generally useful form of camera is
one with bellows extending some thirty inches, and capable of
carrying both quarter and half sized plates, horizontally or
vertically. My box is a square one, carrying plates from the 4-4
size (6-1/2×8-1/2 inches) down to 3-1/4×4-1/4. The bellows are in
three divisions, extending fully four feet, giving altogether nearly
six feet from the microscopic stage [p229] to the focussing
screen when fully drawn out. It is very carefully made of hard wood,
and the framework carrying the bellows moves with the utmost
smoothness upon its Λ-shaped runners. A short cone front receives
the microscope tube, and all extraneous light is shut out by a
wrapping of black velveteen around the opening, secured by a rubber
band. The focussing screen (which is only used for arranging the
object in the centre of the field or plate, and is entirely
removable) has a space of the precise size and shape of a lantern
slide pencilled in its centre, as a guide to the making of quarter
plate negatives. An object filling this space on the ground glass
will necessarily occupy the same position on the quarter plate when
the holder containing the latter is made to take the place of the
focussing screen.
As stated, the ground glass is used only for containing the object,
and for coarse adjustment of focus. It is impossible to grind it
finely enough to accurately focus any delicate tissue with high or
even moderate powers. Many suggestions have been made and devices
used, but all I have tried proved unsatisfactory, until the
following appeared in an English journal:
An evenly coated gelatine plate is to be exposed to a flood of white
light, developed to a very slight intensity--a mere smokiness--fixed
and washed as usual, then bleached with mercury, washed and dried.
The result is the most perfect focussing surface imaginable. To use,
remove the focussing screen from the camera and replace it with the
plate holder from which the slides have been removed. The bleached
gelatine plate (which of course must be of proper size to fit the
holder) is to be placed therein with the film side [p230] toward
the microscope. Now place a focussing glass against the back of the
plate, apply the eye to it and adjust the focus by a delicate pull
of the cord. Nothing can be more satisfactory. When the proper
adjustment is determined upon, secure it from possible change during
exposure, by winding the cord once or twice over the screw eye at
the rear of the camera frame.
The plate holder (single) opens at the back to receive the plate,
and being square, like the camera box, admits of the negative being
taken vertically or horizontally, as may be desired, a matter of no
small importance in many cases.
This feature, in connection with the revolving stage, enables one to
place any object upon the plate in the best position for printing.
The full size of the plate carried by my holders is 6-1/2×8-1/2
inches, but by the employment of removeable rabbett kits, plates
5×7, 4×5 and 3-1/4×4-1/4 can be used. It is desirable to have two or
three additional holders as time savers.
For our work to possess any real uniformity or value, it is
absolutely necessary that the magnifying power used with each
exposure should be ascertained and noted. To make measurements each
time would be a waste of that precious commodity, to avoid which I
have adopted the following device:
The bed or framework upon which the bellows extends, is divided into
spaces of one inch, and the same plainly marked thereon. Suppose
this extension is thirty inches, and that by means of the stage
micro-meter we find the one and one-half inch objective to magnify
twenty diameters with the bellows closed and fifty fully extended.
This gives the value of one diameter [p231] to each inch of
bellows drawn out, and if the magnified object reaches the desired
dimensions, at, say twenty inches extension, we at once know the
power to be forty diameters. Suppose, again, a power of one-fifth is
being used, magnifying 125 diameters with bellows closed, and 275
with the same extended thirty inches; this gives a value of five
diameters to each inch, and if the desired size is reached at
twenty-five inches extension, we have a power employed of 250
diameters. By making these measurements (closed and open) for each
objective habitually used, and recording the same in our note-book,
an accurate and readily adjustable table of measurements is ready
for all future work.
We now pass (thirdly) to the important subject of illumination, in
many respects the most important in photo-micrographic work.
It goes without saying that to the favored few, whose time and means
permit the harnessing of the sun's rays in their service, no
artificial light can be produced which will quite take the place of
the great luminary, but for the many, some other means must be
sought.
When the promised days arrive that are to place in every man's house
electric lights, cheaper than gas and as easily controlled, the
problem will be satisfactorily solved; but at present we must
content ourselves with prosaic kerosene, seeking only for the most
practical method of utilizing its brilliant light.
After many experiments I have settled upon a flat, broad-based lamp
of good oil-holding capacity, which is attached to a heavy retort
stand with movable arm, enabling it to be carried to any desired
height. A duplex burner and a tall chimney, producing perfect [p232]
combustion, afford a light of thirty candle power, rendering
possible the following average exposures with the plates I am now
using, the variation in time being caused by differing densities and
actinism of the tissues or substances to be photographed:
1-1/2 inch objective 3 to 5 seconds.
2/3 inch objective 7 to 90 "
4/10 inch objective 1/2 to 3 minutes.
1/5 inch objective 2 to 7 "
1/10 inch objective 4 to 10 "
This lamp produces sufficient light for use with the highest powers,
requiring the employment of a diaphragm to reduce its intensity. Too
brilliant an illumination, flooding the field with diffusive rays,
will almost certainly produce a blurred and indistinct image.
Microscope, camera and lamp being ready for use, it becomes
necessary to provide a suitable support for each, in order to form a
complete working outfit. Some writers have described and illustrated
their apparatus as being carried upon two or more separate tables or
trestles. This I have found to be a most objectionable method in our
city houses, since the vibration produced by passing vehicles is
unevenly communicated to camera and microscope, producing inevitably
a disturbed or blurred image, especially when high powers are
employed. By adopting, however, a long, solid platform, carrying all
the separate parts of the apparatus, this objectionable feature is
removed, any vibration is communicated to camera and microscope
alike, and there is no blurring of the resulting image whatever. In
my apparatus this platform is about five feet in length, with a
raised portion at one end, [p233] upon which the camera is firmly
clamped by a milled head operated from beneath. The cone front
extends toward the platform, upon which the microscope (inclined
horizontally) is secured at such a height that its tube precisely
enters the middle of the cone. Thus, a straight line drawn from the
object upon the stage through the microscope tube and camera should
fall upon the centre of the focussing screen, and if all the parts
are properly put together this will be the result, and the whole
field will be equally illuminated when the lamp is placed in
position.
The stand of the latter is not secured to the platform, but is left
free to be moved about and placed nearer to or farther from the
microscope, as may be found necessary to secure the best results. A
bull's-eye condenser upon a movable stand placed between the lamp
and the stage of the microscope, is necessary to concentrate the
light upon the object, or upon the achromatic condenser when the
latter is used and to aid in the equal illumination of the field.
The latter is of great importance, since the result of an uneven
illumination will be a negative of differing densities in different
parts, marring its beauty, and at times rendering it utterly
worthless. Too much stress cannot be placed upon the necessity of
securing an even, brilliant light of proper intensity for the object
under examination.
The arranging of the object in the centre of the field (under a low
power) and the coarse adjustment of focus are done with the bellows
tightly closed, which brings the focussing screen so near to the
microscope that, whilst the operator sees the object thereon, his
hand can readily reach the milled head, controlling the [p234]
stage and other movements. But when the bellows is extended to the
length affording the desired magnification it will be found that the
object, whilst retaining its central position, has lost its
sharpness, necessitating a final and careful focussing which is no
longer possible in the same manner as before, since the ground glass
and microscope are so widely separated that the one cannot be
reached whilst looking into the others. Some special device becomes
necessary in this emergency, and many have been made, mostly
complicated and costly. The method I have adopted is the old and
simple one of a fine cord passing around the periphery of the milled
head controlling the fine adjustment, in a groove cut for that
purpose, thence through a series of screw eyes to the rear of the
frame-work carrying the bellows extension, where it is kept taut by
a couple of small leaden weights.
The slightest tension upon this cord causes a corresponding movement
of the fine adjustment, and nothing can exceed the delicacy of its
working or its freedom from derangement. A fine fishing, line makes
an admirable cord for this purpose.
Having sketched the various portions of the apparatus and brought
the whole together into complete form, a few random extracts from my
note-book may not prove uninteresting or valueless to some who are
working in this direction.
Gelatine plates for photo-micrography. The requisites of a perfect
plate are, great sensitiveness, combined with extreme latitude in
time of exposure, and density in development. It should be of a fine
texture, showing clearly the most delicate lines and markings, [p235]
evenly coated and free from spots or blemishes of any sort.[2]
[2] The Stanley dry plate has all these qualities in the highest
perfection.
It is most provoking to have an otherwise perfect negative marred or
ruined by opaque or transparent spots appearing in its most
important portions.
For development, ferrous oxalate or alkaline pyro are equally
useful, and either may be employed, as suits the fancy or
convenience of the operator. My own preference is for the latter,
and I always use it with ammonia well restrained, having had no
success with either soda or potash in this class of work. For all
objects possessing much color it is best to continue the development
until full density is obtained, but for very thin or transparent
subjects, such as diatones or unstained vegetable tissues, it is far
better to stop the development as soon as all details are out and
resort to after-intensification, for which purpose bleaching with
mercury followed by a ten per cent. bath of sulphite of soda will be
found eminently satisfactory. I always use the alum bath, and
invariably secure a clean negative of a cool gray color, resembling
iron development.
Printing. The best ready sensitized paper can be depended on for
producing satisfactory prints, showing the most delicate lines and
markings of diatones and turning to any desired shade. Avoid
over-printing, wash but slightly in two changes of water, the last
slightly acidulated with acetic acid, and use an acetate of soda
toning bath. Undesirable portions of the negative may be stopped out
with a mat of suitable [p236] shape. Any ordinary cabinet card
makes a neat mount of convenient size, upon which may be written the
name of the object or specimen objective and magnification employed,
and any other matter referring to print or negative which may be
necessary to note.
A carefully kept note-book is a most important aid to the worker in
photo-micrography, and it should contain all his failures and
successes alike. Notes should be made of the specimen objective,
magnification, exposure, plate and developer, which, carefully
studied, will almost certainly enable him to secure a success with
each exposure.
Lantern Slides.--It is well to make the negative as far as possible
of a suitable size for producing the positive by contact printing,
which is convenient and satisfactory, though there can be no doubt
that reproduction in the camera affords better results. Slow
gelatino bromide plates, such as Carbutt makes for this purpose,
produce very satisfactory work, but the chlorides are so far
superior that there can be little doubt of their being exclusively
employed for positives in future. There is a richness of tone,
combined with great transparency in the shadows and clear glass in
the high lights, quite unattainable with bromide emulsions, and
rivaling the best wet work.
* * * * *
Opaque Objects.--These may be photographed by the light from a lamp
quite as well as transparent ones, through which the light is
thrown, a matter of which I have thus far spoken only. A strong
illumination must be obtained by the employment of a suitable
bull's-eye condenser or a silvered reflector. The [p237] most
satisfactory method, however, is by sunlight, allowing its direct
rays to fall upon the object without the intervention of any
condenser whatever. Very short exposures suffice with such
illumination, varied only by the reflecting capacities of the object
itself. A successfully exposed plate of this class of objects will
give a print of the subject, standing out most brilliantly upon a
black ground.
To secure the best results, most objects should be specially mounted
for photographing. Some are best in a resinous medium, such as
Canada Balsam, but most tissues are obscured or entirely obliterated
in this medium. So far as possible all preparations should be
mounted in a fluid of some description that will distinctly render
visible many tissues and markings which would be lost in balsam.
This subject, however, is one of so great extent that it would
require a special paper, and I merely refer to it now because of its
importance, hoping at some future time to enlarge more fully upon
it.
You will notice that my remarks have been confined to work that may
be done with objectives of low or moderately high powers, in no case
exceeding one thousand diameters, having preferred to speak only of
that which I have demonstrated by actual work as being practical.
From recent experiments I am fully convinced that the lamp light,
such as I have described, is capable of producing satisfaction, work
with very much higher powers, and shall hope, at no distant day, to
show prints made from magnification of not less than two thousand
diameters, that will be satisfactory in all respects.