RESOLVING POWER OF PLATES 217 



working, and have a comparatively small turbidity 

 factor, and the ratio of the two factors is high. Rapid 

 plates, as a rule, show a greater spreading of the light 

 action consequent on a lower development factor and 

 greater turbidity factor. The spreading does not, 

 however, depend on the size of the grain of the plate ; 

 some fine-grained plates give a greater spreading than 

 others with a coarser grain. 



The sharpness with which detail is reproduced also 

 depends on exposure. With rapid exposures there is 

 less spreading than with more prolonged, but a regular 

 alternation of black and white elements, for which a given 

 short exposure would be equally good over the whole 

 surface, is not often encountered in photomicrograph}^. 

 More generally it is necessary to represent fine detail 

 against a light background in one part of the preparation, 

 and at the same time detail in another part which is 

 thick or coloured. An exposure, small enough to prevent 

 spreading in the former, would probably give no sign of 

 detail in the dense part. And a correct exposure for 

 the latter would hopelessly blur, or entirely obliterate, 

 the fine detail in the thin part or against the background 

 by the encroachment of the high lights over it. 



Halation. This term is generally understood to cover 

 the obliteration of fine detail in this way, and the spread- 

 ing of the effects of light action over the adjacent shadow 

 areas. It also refers to a second action on the back of 

 the film, caused by light which reaches the back of the plate, 

 where it is reflected at the glass and air surface back again 

 through the glass to the under side of the film, producing 

 there a secondary image on either side of the primary one. 



Fig. 68 includes several photomicrographs of the 

 nearly closed slit of a spectroscope taken by a 1J" achro- 

 matic objective on one plate, and originally published 

 in the Amateur Photographer. Seven exposures were 

 given in the ratio 1 ; 2 ; 4 : 8 : 16 : 32 : 64, and the 



