SCKEENS PLATES EXPOSUKE 217 



transmitted at opposite ends of the spectrum. To obtain 

 the best results in this case, a screen as nearly as possible 

 monochromatic green should be used. 



A general rule to obtain the utmost contrast may be 

 formulated for either double-stained specimens, or those stained 

 with dyes which transmit light from widely separated portions 

 of the spectrum. In such cases it is a safe rule to use a screen 

 which transmits light that is intermediate in its position in the 

 spectrum between the two colours it is desired to photograph. 

 Since the spectrum colours occur in the order violet, blue, green, 

 orange, yellow, red, it follows that any two of them may be 

 photographed to secure the greatest contrast in each case by 

 using a colour intermediate between them. Should the two 

 colours transmitted lie together in the spectrum, then the 

 screen used should be as far on either side as possible. 



It may be that the object is not to secure the greatest 

 possible contrast, but to reduce it ; although the occasions on 

 which this happens are not so numerous. In such a case, 

 instead of selecting a Screen which is intermediate in position 

 in the spectrum between the two colours, one should be chosen 

 which transmits the colour in the object which is the most 

 active photographically. If therefore it were desired to 

 photograph a specimen transmitting red and violet, and to 

 reduce the contrast between them as much as possible, a blue 

 or^violet filter might be used ; the resulting photograph would 

 then be a reproduction with a minimum of contrast. The 

 latter condition sometimes arises in preparations that are 

 very deeply stained, or which do not transmit very much 

 light. 



A very efficient series of monochromatic filters can be 

 obtained from Messrs. Wratten & Wainwright. These niters 

 are designed to pass special regions of the spectrum ; the regions 

 are as narrow as possible, just overlapping each other, and the 

 transmission in other regions is reduced to a minimum. The 

 absorption in the region of maximum transmission is designed 

 to be as small as possible, but is still considerable ; and the 

 makers state that the complete transmission is only about one- 

 thirtieth of the incident light even in the region of maximum 

 transmission. These filters do all that is claimed for them, the 

 only point being that in securing such high efficiency a 



