EXPOSURE 175 



The variation of these factors with the nature of the 

 illuminant is very noticeable. Liquid screens of the same 

 spectral transmission have, as a rule, lower factors. 



Unit of Exposure for the Above Tables. To determine 

 the exposure for any object under definite conditions, 

 make an experiment with a good clear specimen, using an 

 oil lamp and no screen at 100 magnifications, with an 

 objective of -5 N.A. on an " M " plate. Suppose that 

 10 seconds proves to be the correct exposure. 



Then for any other conditions, so far as they are covered 

 by the tables, the exposure required is 



10 seconds x factor for N.A. x factor for light 



X factor for magnification X screen factor. 



To take an example. An object is to be photographed 

 with a J" objective of N.A. -85, and a low eyepiece, giving 

 a magnification together of 250 times. Green light by 

 screen B, 1 -ampere Nernst lamp, and " M " plate used. 



The exposure will be 



10 seconds x J X yV X 6 x 1 2 

 = 20 seconds. 



This method of working out exposures from a con- 

 sideration of the individual factors governing it is rather 

 complicated, but it is the most satisfactory method when 

 making any change in arrangement. The basis is a 

 definite exposure determined under the conditions 

 usually employed. As a rule a worker does not employ 

 a wide range of different conditions, and it is fairly easy to 

 arrive at a unit of exposure for the plate commonly used 

 with each of the screens employed ; the other factors for 

 magnification and N.A. are then very rapidly made use 

 of. More frequently still, previous experience has de- 

 cided a correct exposure under certain conditions, and 

 the alteration made necessary by a change in one of them 

 is all that is required. 



