scopes of the highest perfection mechanically, and closely approaching that standard, optically. It may be said in this connection also that though apparatus of excellent quality is thus readily obtained, yet a reasonably accurate knowledge of the optical principles involved and of photographic manipulation is quite necessary to insure satisfactory results. In fact it is the writer's conclusion, after some years' experience connected with the various branches of scientific photography, that no more difficult problem has presented itself than the production of a thoroughly satisfactory photomicrograph of a specimen magnified to 2,000 diameters.
Probably many failures in the work are directly traceable to the use of inferior microscopes, and a few points noticed here may be interesting and possibly instructive.
It would seem hardly necessary to mention the elementary principle in optics, that the spectrum resulting from the resolution of white light is divided into three more or less distinct parts, according to wave length and effect upon matter. Beginning with the longest wave length, we have the infra red and red, or heat portion; then the yellow and green, or light (visual) portion; and finally the blue and violet, or chemical portion. Simple lenses, being of prismatic origin in manufacture, refract white light in such a way as to resolve it into its component colors, and the wave length of the red being greater, it suffers less refraction than the yellow, which in turn is refracted less than the blue; and thus the converging rays do not focus at the same point as a whole, and chromatic aberration results. Further, the fact that for any given lens there is a decrease in thickness from center to circumference, results in unequal refraction of the light as a whole and spherical aberration results, which practically means distortion of the image of an object. Both these defects, chromatic and spherical aberration, are reduced to a minimum by the combination of crown and flint glass to correct the former, and a reduction of aperture or multiplicity of lenses to overcome the latter.
It is a well-known fact, however, that the ordinary microscope objectives have what may be called residual chromatic aberration which focuses the light rays a trifle nearer the lens than the chemical rays, and thus when the image is perfectly sharp upon the ground glass of the camera, the chemical rays, which alone are active upon the photographic plate, do not accurately delineate the object. Fraunhofer has shown, that when the portion of the spectrum of greatest intensity upon the retina, that between the yellow and green is expressed by 1,000, the part between the blue and violet is only 31, so the difficulty in focusing the chemical rays is obvious. It actually amounts to focusing carefully the image, and then moving the objective toward the object an indefinite distance,
