252 PHOTOMICROGRAPHY 



with the same transmission as the dark parts of the 

 preparation was indicated. The body of an insect 

 absorbs most of the blue light, and in consequence, under 

 ordinary conditions, is shown in very great contrast to 

 the wings or other parts which transmit blue. If blue 

 light is excluded by a screen, and the photograph taken 

 by red light, the difference of exposure required by the 

 clear and brown parts is much reduced, and both will be 

 properly reproduced. Panchromatic plates are evidently 

 the best to use under the circumstances, and Plate 20 

 shows the difference obtained by photographing in blue 

 and in red light. As a rule, however, orthochromatic plates 

 are quite satisfactory for a well-cleared insect, but detail 

 in the yellow parts will almost invariably be lost, and 

 brown or yellow markings emphasised if a non-colour 

 sensitive plate is used ; both these defects are shown by 

 the photograph of the Window Gnat (Plate 14. A). In 

 extreme cases it may be well to increase the contrast of 

 very delicate preparations by means of a green or blue 

 screen, and this was done for the Flea (Plate 13) and 

 the Louse (Plate 14.B). The Louse was so transparent 

 and the balsam had become so yellow, that it was neces- 

 sary to increase the contrast as much as possible. 



The delicate hairs which surround many insects are 

 easily obliterated by halation, or appear double or sur- 

 rounded by white lines by diffraction (Plate 12), and 

 care must be taken to avoid these defects in the photo- 

 micrographs by using backed plates, and not closing the 

 substage iris too much. 



Preparation of Insects. A good photomicrograph of 

 an insect depends so much on the preparation that the 

 method employed for clearing the specimen and making 

 it as transparent as possible is given. 



Immerse in 10% solution of caustic potash until soft ; 

 this requires times varying from a few hours to weeks, 

 according to the nature of the insect. 



