280 PRACTICAL PHOTO-MICROGRAPHY 



the plate that is exposed in the camera. This is accomplished 

 by coating the plate with a series of niters, which, in the 

 case of the Lumiere autochrome method, consists of minute 

 dyed grains of starch ; or in the Thames or Dufay colour-plate 

 method, is composed of a series of fine accurately ruled coloured 

 lines or bands. Over either of these screens the emulsion is 

 spread so that the exposure takes place through the combined 

 colour-screen, and is visible on development of the plate. In 

 each of these processes the instructions given by the makers are 

 so complete that it would not appear necessary to repeat them 

 in detail. The points that require special consideration in photo- 

 micrography are the source of light and the yellow subsidiary 

 screen through which the photograph is taken. It happens that 

 with daylight, or with any other source of light, that the plates 

 as prepared are still too sensitive to the blue end of the spectrum, 

 so that a yellow-glass screen has to be interposed between 

 the source of light and the object to cut out the unnecessary 

 portion of the light. When daylight is being used these 

 yellow screens, as supplied by the makers, are perfectly efficient ; 

 but with artificial light-sources either an additional screen 

 or a specially prepared one becomes necessary. The source of 

 light should approximate as nearly as possible to daylight, 

 and for this reason acetylene is perhaps the most suitable. 

 With this illuminant the ordinary screen supplied by the 

 manufacturers for use with daylight works well. If an 

 ordinary petroleum lamp is used then excellent results may 

 be obtained without using any additional screen ; in fact, some 

 of the best results that have passed through the writer's hands 

 have been arrived at by this means. 



When using the Nernst electric lamp a special yellow 

 filter must be used. It consists of a yellow and blue screen 

 which are made separately, and then bound together. The 

 formulae are as follows : 



Yellow Screen. 



Gelatin solution in distilled water, 1 in 15 . 40 c.c. 

 Tartrazine solution, 1 in 2500 . . . 3 c.c. 

 Aesculin, dissolved in 37 c.c. of water, with 



the addition of 3 drops of ammonia . O'l grm. 



8 c.c. of this solution are used per square decimetre of surface. 



