236 PHOTOMICROGRAPHY 



A series of glass or fabric screens should be obtained, 

 sliding in a frame over the window, if daylight is to be used, 

 or fitting into the lamp. A canary- yellow can be used 

 for gaslight papers, but ordinary plates must be developed 

 in ruby light. Orthochromatic plates may require two 

 thicknesses of ruby glass, or glass and fabric, and pan- 

 chromatic plates can only be developed in darkness, or 

 at most in a dull green light adjusted to their region of 

 least sensitiveness. A properly made safelight by such 

 firms as Wratten and Wainwright or Sanger-Shepherd 

 can be relied on not to fog the plates for which it is 

 adjusted, while an ordinary piece of red glass or fabric 

 which does not pass nearly so much light might be quite 

 unsafe. In any case it is advisable to expose colour- 

 sensitive plates as little as possible to light of any kind ; 

 the dishes should be kept covered during development, 

 and the plates examined as little as possible. 



The difficulty of developing in the dim light used for 

 orange and red sensitive plates may be partially over- 

 come by grading the safelight. Wratten's No. 3 safelight 

 for panchromatic plates, for instance, is made up of two 

 glasses, one coated with a green and the other with a 

 yellow film, bound up face to face with a piece of green 

 paper between. The three transmit a very feeble green 

 light, which is practically of no use unless one remains in 

 the dark room a considerable time for the eyes to become 

 accustomed to it, but a brighter light can be obtained by 

 taking the binding off three edges and cutting a vertical 

 strip about an inch bro off the green paper. The paper 

 is then replaced and the whole bound up again, forming 

 a screen with a narrow strip down one side which trans- 

 mits considerably more light than the rest. A panchro- 

 matic plate would be fogged if exposed more than 

 momentarily to this light, but by keeping that part 

 shaded, and only using it for rapid examinations of the 

 plate from time to time, or for reading the watch, de- 



