By means of photography, however, we get faithful images of the objects in a few seconds.
In practice we require three essential things. These are : A camera, a microscope, and a lamp. Many arrangements have been devised for photographing microscopic objects. The simplest is a small camera made to fit on to the upper end of an ordinary microscope in place of the eyepiece. Fig. 333 illustrates a more perfect form of photo-micrographic camera.
The microscope used should he a firmly-built solid monocular with a fine adjustment. The light used must be accurately centered, otherwise one side of the object will be dark and fuzzy.
The tube of the microscope should be coated with a dead-black varnish or lined with dull black paper.
The light can be an ordinary microscope lamp burning best paraffin oil containing a little camphor dissolved in it to increase the brilliancy of the illumination.
Another indispensible adjunct is a substage condenser fitted with a revolving series of stops.
Before attempting to focus, the lamp should be allowed to burn for ten minutes, so that the whole instrument may become thoroughly warmed throughout, otherwise the expansion will alter the focus very considerably. Great care must be taken in focusing the object as sharply as pos sible. Charters White recommends the removal of the ordinary glass focusing screen, replacing it by a plain glass, having some fine lines drawn across it with a writing diamond, these lines being placed next the objective. The focusing glass is then set until these lines are sharply defined. These lines will closely approximate to the plane of the emulsion on the sensitive plate, therefore if the details of the object are in focus with the lines on the focusing screen, the prob ability will be in favor of a sharp image on the negative.
With regard to the exposure. This varies in proportion to the magnifying power of the objective used, and the color of the object itself. The times in the following table (by Walmsley) may be considered as approximate, but variable with the color and general character of the object to be photographed.
II inch objective 3'45 secondsinch objective 7 go seconds inch objective 1'3 seconds1 inch objective 2'7 seconds inch objective seconds Experience only will teach the correct exposure, however.
Mr. A. A. Adee, writing in the "American Annual " for 1895, gives some valuable hints to those desirous of studying this fascinating branch of our art. He says: " While the attain ment of the highest results in microscopic photography demands signal proficiency and an outlay running far into the hundreds, any possessor of a fairly good compound microscope and a small camera can produce pictures at once interesting and meritorious. It is a mistake to suppose that an elaborate stand with modern apochromatic objectives and illumination by heliostat or calcium light is requisite, except for the most advanced bacteriological and diatomic work. Indeed, it would be a positive drawback to the mere amusement seeker to have so expensive an outfit, for he must needs undergo a year or two of professional tuition or wasted experiment before reach ing critical success.
" I long shared the general belief and postponed any essays in this tempting direction until I could get an eight hundred dollar apparatus, thereby missing several winters of singularly interesting pastime. Having a good Nachet stand with mechanical stage and a well selected set of objectives, and also a discarded 4 x 5 camera, it at last occurred to me to put them together and try conclusions therewith. A stout base-board, four feet long and six inches wide, slotted along the middle and set on turned four-inch legs, gave a good beginning. To this five suitable blocks of wood were fitted, each having a screw-pin passing through the basal slot and fastening by a thumb-nut. To the rearmost block the camera was pivoted, a paste-board funnel with black-velvet lining being attached to the front, so as to allow some two feet draw. To the next block the microscope was firmly clamped, the eye-piece being, of course, removed, and the open tube being joined to the little end of the funnel by a light-tight sleeve of velvet. The third bore a big bull's-eye condenser, and the fourth a heat and light filter. The heat-filter gave a world of trouble, but a six-inch ring cut out of half-inch all-rubber steam-packing, with plates of stout glass generously cemented to each side by means of a paste of red lead and gold size, supplied a water-tight cell, a hole being bored in the rubber wall to admit of filling it. A flat-sided pocket flask of clear glass, holding half a pint. was afterwards found to do nearly as well. It should be filled with a strong (4o per cent.) solution of copper nitrate, to which a grain of chromic acid is added, giving a practically monochromatic blue-green light that does not call for the long lamp light exposure needed by the yellower liquid usually employed for modifying direct sunlight. One can't get along without a heat-absorbing cell, even if it only contains pure water; if it be omitted the warmth of the condensed lamp rays will often melt and ruin a balsam-mounted slide, or, at least, cause it to run, thereby shifting the object and yielding a panoramic effect, which is disheartening in a negative. On the last block came the lamp, a common, low-bodied kerosene hand-lamp, with a two-inch flat wick and a roomy chimney of the Rochester pattern. It was set with the edge of the flame toward the microscope, and the inside of the chimney away from the flame, was coated with a round patch of smoke from a wax gas taper, so as to avoid reflection. A reflector behind the source of light ought not to be used, for the beam is thereby so broken up as to throw a distorted pencil, and what is gained in light is lost in definition. Care was taken to so adjust the thickness of each supporting block that the optical axis of the whole system ran truly from the flame, through the tube of the microscope, to the focusing screen. To the center of the latter, on the ground side, a one-inch cover circle was cemented with balsam in order to focus the image on the transparent spot by means of a set lens.