CH. VIII] PHOTO-MICROGRAPHY 215 



room even to the light of the developing lantern. The more nearly the plate 

 can be kept from all the light, except that acting during the exposure in the 

 camera, the more satisfactory will be the resulting negative. 



\ 291. Requirements for Successful Photo-Micrography. — Successful vis- 

 ual images may be obtained in two ways, (§ \ 118-119, 157-159), viz: by mount- 

 ing the object in a medium whose refractive index differs markedly from the 

 object; or by staining the object so that it has a markedly different color from 

 the mounting medium. When the two methods are combined and the object 

 differs both in refractive index and in color from the mounting medium the 

 visual images obtained through the microscope are most satisfactory. 



In photography the difference in refractive index between object and 

 surrounding medium is of the same importance as for ordinary observation, 

 and as with the eye, the greater the difference the bolder the outline ( § 157). 

 But difference in color of object and mounting medium does not ensure a good 

 photographic image. This is because the wave lengths of light producing 

 the different colors are not all equally effective in producing a photograph. 

 The visually brilliant long waves of red, orange, yellow and green are far less 

 effective in producing a picture than the shorter waves of blue, indigo and 

 violet. In a word the end of the spectrum brightest to the eye is least effec- 

 tive for producing a photograph, i.e. the sensitiveness of the eye and the pho- 

 tographic plate are inverted or complementary. 



- As stated above ( \ 290) color sensitized plates have been produced to meet 

 a part of the difficulty. To further perfect the photographic image and make 

 it correspond more closely with the light effects of the visual image, color- 

 screens or filters have been devised whereby the light transmitted by the 

 specimen is partly or wholly eliminated from the light illuminating it. If the 

 color screen wholly eliminates the light which the object transmits then of 

 course its color to the eye is eliminated and the object appears black, no mat- 

 ter how brilliant the illumination. It is also black to the photographic plate, 

 and shows as a black object in the picture. 



The dyes used in staining microscopic preparations differ not only in the 

 wave length of light they allow to pass through the object, but they also differ 

 in the amount of opacity to all light which they give to the specimen. This 

 is a valuable feature for photography. For example hematoxylin transmits 

 much actinic light, but it also renders the object more or less opaque lo all 

 light and hence specimens well stained in hematoxylin usually give good 

 photographs. Carmine stained specimens also give good photographs because 

 they are rendered slightly opaque to all light, but principally because the red 

 stain is especially opaque to the short waves. If one could select stains for all 

 objects which would greatly lessen the passage of all light, the amount cutout 

 depending on the density of the specimen in different parts, and also elimi- 

 nate the greater number of the short waves, it would be easy to produce good 

 photo-micrographs. Where stains with the above qualities cannot be employed 

 it is necessary not only to use isochromatic plates but a proper color-screen. 



I 292. Color Screens. — For the intelligent use of color-screens it must be 

 borne in mind that colored objects appear colored to the eye because they 



