7 8 ELEMENTARY PHOTO -MICROGRAPHY. 



in the blue- violet. Thus a sensation of light blue 

 is produced by a mixture of green light and blue- 

 violet light falling upon the eye, the red light 

 being more or less absent, having been absorbed 

 from the white light by the object, which appears 

 to be coloured light blue. 



"If a colour is to be rendered as black as 

 possible, then it must be viewed or photographed 

 by light which is completely absorbed by that 

 colour." 



In other words the light filter should be of a 

 complementary colour to that of the object. 



Another rule for procedure deals with the case 

 where contrast is required, not against the back- 

 ground but within the object itself. 



" A good case of this is the photography of an 

 unstained section of whalebone. This is of a 

 yellow colour, and shows ample detail to the eye, 

 but it completely absorbs blue-violet light ; and 

 if it is photographed on an ordinary plate sensitive 

 only to blue violet light, then it shows far too 

 much contrast, appearing as a black detail-less 

 mass against the background, and presenting an 

 exaggerated example of the loss of detail which 

 has already been noted in the eosine section 

 photographed by light which it completely absorbs. 

 The proper procedure in this case is to photo- 

 graph the object by the light which it transmits. 



