56 The Microscope 



tures of a slide. This is done with the aid of colored glass filters inserted 

 in the system. These filters remove from the illuminating system those 

 colors which it is desired to emphasize, as dark shades, in the finished 

 print. Let us take, as an example, the stem of Aristolachia shown in Figs. 

 69, 70, and 71. In this, the nuclei and xylem are stained red with safranin. 

 The cell walls are stained green. A well-balanced panchromatic film will 

 record the red and green as having equal values of gray. This is seen in 



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69 



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Sm 



70 



Y v rs • . ■ *4*c i 



71 



Figs. 69, 70, and 71. The effect of color filters in photomicrography. The object pho- 

 tographed is a vascular bundle in the stem of Aristolochia stained by the method of 

 Johansen described elsewhere in this book. The xylem and nuclei are scarlet. Other 

 cell walls and cell contents are green. Fig. 69 shows a photograph on a properly bal- 

 anced panchromatic film which renders the various colors in the proper intensity of 

 gray. Fig. 71 shows the result of inserting a green filter into the system to emphasize, 

 the xylem. Fig. 70 shows the result of using a red filter which emphasizes the cell 

 walls and cell contents. 



Fig. 69. The insertion of a green filter into the system will remove red 

 from the light beam so that the xylem, red in the slide, will appear black 

 in the print ( Fig. 71 ) . This, although a conventional thing to do, is very 

 doubtfully an improvement. A medium-red filter will remove green from 

 the illuminant so that the cell walls ( Fig. 70 ) will appear brilliantly de- 

 fined. There is no limit to the number of filters that may be purchased, 

 but a medium green ( Wratten N or No. 61 ) and a medium red ( Wratten 

 A or No. 25) are all that were used in the preparation of illustrations for 

 this book. 



Exposure. No method more useful than trial and error has yet been 

 developed for estimating the necessary exposure in photomicrography. 

 Photoelectric exposure meters are intended to record the light reflected 

 from a solid object to be photographed. A photomicrograph records the 

 light transmitted through an object. Photoelectric meters "adapted" for 



