42 PRACTICAL PHOTO-MICROGRAPHY. 



crest to crest, from roughly 36 l do th to ggoooth of an inch ; 

 and of these varying wave-lengths those which exercise the 

 greatest chemical action measure about -^^tli of an inch, 

 and are what we call "violet" in color. If, then, we can cut 

 off all rays except those which exercise strong chemical action, 

 we shall reduce the general actinic force of the white ray, for 

 all visible rays have some actinism, but we shall have less 

 confusion among the rays producing the photographic image, 

 and so our optical apparatus will probably be used at its best, 

 especially if our lenses are corrected for the rays which pro- 

 duce the best visual effect, as lenses naturally are when in- 

 tended for ocular observation only. Two methods were in 

 vogue for this passing of actinic rays alone : one was the use 

 of a cell containing cupric ammonio-sulphate dissolved in 

 water, which makes a blue solution, and one spectroscopically 

 suitable for the purpose aimed at ; * the other plan consisted 

 in the use of a prism which broke up the white ray into its 

 component parts, the blue part alone being allowed to pass 

 through the microscope. A " diffraction grating" would yield 

 a still purer spectrum, but so far as we know has not been used 

 for this purpose. In each case the solar ray was passed through 

 the monochromatising medium before it reached the object. 



Diffused daylight may be utilized by reflection from white 

 cloud or uniform blue sky; but after repeated and careful 

 experiments the writer can not recommend this system of 

 illumination as likely to prove satisfactory to the serious photo- 

 micrographer. If white cloud or homogeneous sky can be 

 depended on, the ordinary plane mirror of the microscope, if 

 of ample size, will answer ; all rays in this case being practi- 

 cally parallel at their impact on the mirror. Another plan, 

 tested by the writer with greater success, is to omit the mirror 

 and to replace it by a white surface, as very fine filter or blot- 

 ting paper, but not a shiny surface as sized paper or opal glass 

 The white surface is to be inclined in a suitable direction at an 

 angle of about 45 deg. to the axis of the optical system. 



*This solution is, in the writer's experience, usually far from monochro- 

 matic and decidedly inferior to certain qualities of cobalt blue glass for 

 this purpose. 



