i44 POPULAR SCIENCE MONTHLY. 



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 prin- 

 ciple in optics, that the spectrum resulting from the resolution of 

 white light is divided into three more or less distinct parts, accord- 

 ing 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 micro- 

 scope objectives have what may be called residual chromatic aber- 

 ration 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 de- 

 lineate the object. Fraunhofer has shown, that when the por- 

 tion 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, 



