Ch. VII] PHOTOGRAPHY WITH COLOR SCREENS 235 



(S) If transmitted light is used some parts of the object must be 

 transparent or translucent and other parts opaque. The opaque parts 

 will then appear dark and the transparent or translucent parts light. 



Two, four, and five might properly be called absorption images. 



§ 365. Photography is admirably adapted to represent the visual 

 appearances of both naked eye and of microscopic objects. There 

 is only one difficulty which is really serious, and that is in the proper 

 representation in black and white of the various colors. 



This difficulty is inherent in the sensitiveness of the eye to colors 

 and the unlike sensitiveness of the photographic plate to the same 

 colors. If both were equally and similarly sensitive, then the photo- 

 graphic representation of color in shades or tones of black and white 

 would have the same brightness as the different colors to the eye. 

 But the eye has its maximum sensitiveness in the green (fig. 139), 

 while the photographic plate has almost all of its sensitiveness in 

 the violet-blue end of the spectrum. Indeed it is sensitive to a part 

 of the ultra violet which is wholly dark to the eye. Hence the photo- 

 graph represents the brilliant red-orange-yeUow-green image seen by 

 the eye as dark, while the relatively dark violet-blue to the eye is 

 rendered white by the photographic plate. The photographic image 

 of colored objects is then a kind of negative of the same image to the 

 eye. This has made the use of photography unsatisfactory where 

 objects have color, and most objects in nature are colored more or 

 less; and one of the greatest triumphs of microscopic science has been 

 the differentiation of details of structure by selective staining. 



From the earliest history of photography the inabihty to render 

 the colors properly or in actual colors has been greatly deplored. To 

 give the proper brightness in tones of black and white to colored 

 objects two things had to be attained: 



(i) The photographic plates which were originally sensitive only 

 to the violet-blue end of the spectrum had to be rendered sensitive 

 to the other colors. The first step was in getting plates sensitive to 

 the spectrum as far as the yellow. These are the so-called Isochro- 

 matic or Orthochromatic plates. The final step was to get plates 

 sensitive to all the colors of the spectrum, including the orange and 

 red. These are known as Panchromatic or Spectrum plates. 



