216 PHOTOMICROGRAPHY 



Resolving Power of Plates. Resolution depends mainly 

 on the optical arrangements of the photomicrographic 

 apparatus, but to a certain extent the nature of the plate 

 limits the fineness of detail that can be sharply reproduced. 

 The transition also from dark to light in the photo- 

 graphic reproduction of lines and points on a light ground, 

 or bright detail on a dark ground, is more or less gradual 

 and indefinite, and causes very minute, closely packed 

 lines, etc., to merge one into another. Dr. C. E. K. 

 Mees examined this question and found that the " Re- 

 solving Power of Plates " increases as the size of the 

 grains in the emulsion diminishes. An extra-rapid plate 

 with grains -0015 to -004 mm. in diameter could just 

 separate lines -030 mm. apart, while a process plate, in 

 which the grains are -0010 to -0015 mm. in diameter, 

 resolved lines -018 mm. apart. The increase in resolving 

 power does not go much beyond this, as smaller grains 

 set up diffractions which act in the opposite direction. 



More recently , Dr . E . G oldber g has investigated the spread- 

 ing of the light action from the lines and points of detail 

 into the surrounding parts of the plate, and in a paper com- 

 municated to the Royal Photographic Society stated that 

 the " Sharpness of a Plate " depended on the ratio of the 

 " Development factor " to the " Turbidity factor " of the 

 plate. The first of these factors expresses the gradations 

 produced by the plate, the second is the ratio of the light 

 scattered in the film to that falling on it. The greatest sharp- 

 ness of image is given by the plate which has the smallest 

 turbidity factor and the greatest development factor. 



The Lippmann grainless plates have a low develop- 

 ment factor that is, are very soft-working or give flat 

 gradations, but their turbidity factor is so exceedingly 

 small that the quotient of the first factor by the second 

 is so high as to determine a very sharp image. 



Process plates which are distinguished for their 

 capacity to give sharp reproductions are very hard- 



