riioroMicRocK.iriiy .i\i> iiciixic.ii. mickoso^I'V 



1()0 



(•rt'.it brilliancy is not retiiiirccl for this work but rather a difTusrd 

 lii^ht. obtaini'il by nu-ans of intcrjiosing j\ ground glass screen in the 

 ilUnninaling beam. 



The object in photomicrography is to record as clearly and as 

 faithfully as possible the structural characteristics of the specimen. 

 This is accomplisheil by a rentlering of contrast between the structural 

 elements of the specimen antl by intensifying or diminishing this 

 contrast to suit the particular characteristics which^iare to be re- 

 produced to best ailvantage. This cooli'ol of contrast is obtained 

 by control of the color of the light used for illumination. 



.•\ s[x?ctroscopic analysis of the light of the arc shows a continuous 

 siH'Ctrum consisting of three dominant color portions, blue-violet, 

 green and red which pass by gradation to each other; the blue-violet 

 passes by blue and blue-green to green, and the green by yellow and 

 orange to red. 



If an object absorbs some constituent of the white light falling 

 on it then the reflected light will be deficient in this color and as a 

 result the eye will experience the sensation of color. 



The effect on the color of the residual light by blocking out a narrow 

 band at different positions in the spectrum is shown in Fig. 7a. 



Fig. 8 — Diagram representing the spectrum of arc light divided roughly into three 

 dominant bands. 



.-\ simple diagrammatic representation of the visible spectrum is 

 shown in Fig. 8, in which the tri-color (ii\ision is broadly made as 

 follows: 



Blue- Violet 4,000 to 5,000 A.U. 



Green 5,000 to 6,000 " 



Red 0,000 to 7,000 " 



An object which appears red to the e>e when illuminated by white 

 light is absorbing the blue-violet and the green light, antl the bulk of 

 what it reflects or transmits is red. Similarly, an object appears 

 green because it is reflecting or transmitting the green constituents 

 of the spectrum and absorbing the retl and the blue-violet rays. These 

 are simple cases assuming sharp absorptions and ideal conditions, 

 but in the practice of the art of photomicrography we are dealing 

 with gradation in- color and oftentimes the structural characteristics 



