If you place any natural object at a distance from a convex lens, a much smaller image of the same will appear at the focus of the lens. When this small image is reduced to that point that it cannot be seen clearly with the naked eye, and is photographed, it may be called a micro-photograph. On the contrary, if the object is a microscopic one, and is placed at a point very near a convex lens, an enlarged image may be found at a considerable distance from the lens. A photograph of this enlarged image of a microscopic prepared object, is termed a photo-micrograph.
Some confusion among so-called authorities on this subject renders their definition of sufficient importance to be given in this connection.
With the former we have nothing to do, as there can be very little interest attaching to the production of pictures of any object on so small a scale as to require a microscope to look at them.
With the latter, however, it is otherwise, for by the aid of photo-micrography we are enabled to place in permanency for closer study those enlargements of natural objects that are revealed to the human eye by the microscope. The secrets of the composition of the drop of water and a knowledge of its "monstrous and terrific inhabitants," are thus made known.
Previous to the introduction of the sensitive dry plate the practice of photo-micrography was confined almost wholly to those possessed of leisure, and ample means to provide the expensive and complicated instruments necessary, or to savants like Dr. Woodward, backed by the resources of a liberal government supplying apparatus and appliances far beyond the means of the ordinary student of natural science.
The advent of the gelatine dry plate placed the workers in this branch of scientific research more nearly on a level, at least as far as the possession of suitable and efficient instruments are concerned, and one of these workers has published for the instruction of amateurs and others a very valuable article on photomicrography, from which we extract by permission a description of the instruments necessary to the successful practice of this interesting study.
Mr. Walmsley, in the article referred to, says: I will now describe the apparatus I am using at present.
passing unnoticed the various stages by which its comparative perfection has been reached. Four essential parts make up the complete outfit.
A microscope with its object glasses and other accessories.
A camera with long extension bellows.
A lamp for illumination.
A solid platform to carry the entire apparatus.
A microscope with a joint to the body permitting the same to be inclined to a horizontal position may be employed, but the essentials to accurate, comfortable work are as follows:
A heavy, solid stand, perfectly firm in all its positions, with a short body (the 6-inch Continental model is admirable), good coarse adjustment for focus, and a very delicate, fine one, the latter controlled by a large milled head, in the periphery of which a groove is turned to admit the focussing cord, hereafter to be described. A solid, mechanical stage rotating in the optic axis, and a sub-stage moved by rack and pinion, capable of carrying an achromatic condenser, polariscope, etc., are almost indispensable. The objectives should be the very best, especially corrected for photography in all powers under the one-fifth of an inch and of moderate angular aperture.
Perfect corrections of chromatic and spherical aber-ations, with the greatest penetrating and defining powers, are essential features in objectives designed for photography.
A binocular microscope can be used quite as well as a monocular, by simply shifting the prism, but if an instrument is to be selected purposely for photographing, the latter is recommended in preference to the former. In either case, the tube is to be lined with black velvet or flock paper, otherwise there will be a reflection from its inner surface, which will cause a ghost in the negative. An accurately centered double or triple nose-piece, carrying as many objectives of different powers, will also be found an immense convenience, saving much time and labor in choosing the proper object glass for any particular object. I use a triple nose-piece, upon which are always screwed a two-thirds, a four-tenths, and a one-fifth inch objective, being the powers most generally used. If the one-fifth is to be employed in making an exposure, the object is readily brought into the centre of the field under the two-thirds by a simple rotation of the nose-piece; the one-fifth is now brought into position, when the object will be found to occupy the centre of its field of view. Much valuable time, which would otherwise be expended in finding a minute object under so considerable a power, is thus saved.
Most chemical crystals being colorless and nearly invisible under ordinary illumination, require to be examined and photographed by polarized light. For this purpose, the polariscope prisms should be of large size, and both mounted in revolving settings. The lower prism should also fit into a sub-stage, capable of carrying an achromatic condenser above, in order to properly illuminate the field with any objective above the two-thirds inch. The achromatic condenser itself should consist of at least two combinations of different powers and of wide angular apertures. It is always. necessary to employ a condenser of greater power than the objective. If a one-fifth is used, the illumination should be by a four-tenths to get the best results, whilst a one-tenth should be illuminated by a one-fifth. For lower powers, the ordinary bull's-eye condenser, in connection with a diaphragm of varying apertures, will be found all-sufficient.
For the great majority of workers the following powers of object glasses are recommended: Two inch or one and one-half inch, two-third inch, four-tenth inch, one-fifth inch and one-tenth inch, the latter an immersion capable of being used with glycerine, since water evaporates too rapidly for practical use. With these powers, and a camera having thirty inches extension of bellows, a range of powers from about ten to six hundred diameters may be obtained, whilst the addition of an achromatic amplifier will increase the power of the one-tenth to more than one thousand diameters, without perceptible loss of light or definition.
All the foregoing features are combined in the microscope I am using for this work, and, in enumerating them, I have but described that very perfect and complete instrument. We pass now to the second part of our outfit.