For the printing of photomicrographs, smooth gaslight paper seems preferable to all other methods ; the least satisfactory is P. O. P. gelatine, owing to the great loss of detail in toning and fixing. A glossy surface paper must be chosen. For lantern slides, gaslight plates are far the best, and, as few operators would attempt photo-micrographs of larger size than quarter plate, they can generally be made by contact.
Medium- and High -power Work.—The essential outfit consists of a long extension camera, a microscope with object glasses, etc., a good lamp, and last, but not least, a solid platform. Makeshift arrangements, in which the various instruments are brought into contact with piles of books on an ordinary table, only end in disappointment and loss of time. The foundation of all things should be a piece of well-planed board, about 4 ft. long, 12 in. wide, and i in. thick, at one end of which the camera will rest screwed to a box or block of wood, which will raise the centre of the focussing screen to the level of the tube of the microscope when in a horizontal position. During the last twenty years the evolution of the field camera has proceeded on lines which render it most unsuitable for use in connection with the microscope, while the older patterns lack the necessary extension. Unless the operator possesses unusual mechanical and inventive powers, he will find it the cheapest way in the long run to invest in a proper camera, constructed for the purposes of photo-micrography. Day light enlarging cameras can generally be adapted, but are somewhat cumbrous, considering the small size of plate in use and the great additional length of baseboard entailed.
The Microscope.—Any good microscope, with a joint to the body enabling it to be inclined to a horizontal position, may be employed, but for a home-made arrangement a solid stand is essential, perfectly firm in all positions, no matter to what angle the body tube may be inclined. Tube and substage should be fitted with standard thread and gauge of the Royal Microscopic Society, all good modern lenses being made with this standard thread. Cheap and old fashioned instruments are not always corrected for chemical as well as visual focus. Perfect correction of chromatic and spherical aberration, as well as close definition, are indispensable. The tube must be lined with black velvet internally, or at least blacked throughout ; otherwise, when photographing without the eyepiece, there will be reflections from the metal surface, causing bright spots in the centre of the focussing screen.
Source of Light.—Oil lamps are rather to be discouraged, except for low magnifications ; the intensity of light is often variable, exposure is very long, and another objection is the danger of greasy fingers. The arc light has its little erratic ways ; but the Nernst burner will be found more satisfactory where electricity is available. Acetylene is perhaps the best of all, especially if the burners are enclosed with a small round hole for the emission of the light after the manner of an army-signalling apparatus. Incandescent gas is also convenient, and conducive to cleanliness and absence of anxiety with regard to the lighting department. For higher powers it is insufficient, owing to the fact that ground glass has to be interposed between burner and condenser, in order to keep the image of the mantle from producing uneven illumination.
A bull's-eye condenser must be introduced between the source of light and the object glass, in such a way as to evenly illuminate the field. For all powers higher than one inch we must employ a substage condenser, consisting of a series of lenses to concentrate upon the object a wide angle cone of light. When opaque objects have to be illuminated the source of light must be moved to one side, ten inches being usually a convenient distance, and the light focussed upon the object by means of the condenser ; the smaller the image of light, the better for the result.
Focussing with the Microscope.—We will suppose that the object on the stage of the microscope is of transparent character, and that lamp, condenser, and camera have been placed in position. The object is, say, a simple botanical section, and the I in. power will suffice. It is first focussed by means of the eyepiece. The latter is then removed and a piece of white cardboard is held in front of the entrance of the microscope tube, and from 8 to to in. away. An image of the illuminant will be thrown upon the card, and a distance at which this is sharp must be chosen. If the lighting is not uniform over the whole circle, either lamp or condenser is not truly central, and must be readjusted until all is clear and sharp, with the image well defined.