i82 SCIENCE PROGRESS 



reach its maximum value at a comparatively low density. 

 Fine-grain plates have usually high contrast and low turbidity, 

 and therefore give sharp images ; the sharpness, however, is 

 not due primarily to the fineness of grain, as is commonly 

 supposed. The sharpness arising from a random distribution 

 of grains is of a higher order than occurs in real emulsions. 



The resolving power of a photographic plate, on the other 

 hand, depends directly upon the size of grain. Ross finds that 

 the resolving power is proportional to S {d* + «')"*, where S 

 is the sharpness, d \s 2l measure of the size of grain and a is a 

 constant. This formula may be illustrated by the case of an 

 average emulsion with chemical and physical development 

 respectively. The turbidity factor in the two cases is the 

 same, but the contrast factor is much larger for chemical than 

 for physical development. The sharpness under physical 

 development is therefore much less, as can be seen by eye. 

 Nevertheless, the resolving power is greater under physical 

 development, owing to the exceedingly fine grains which result. 

 The resolving power varies with the wave-length, having in 

 general a minimum value in the green region. 



In astronomical work allowance has to be made for the 

 turbidity arising from the optical system, due to the image of 

 a point source not being a geometrical point. The optical 

 turbidity in general overshadows the inherent turbidity of the 

 emulsion, so that then the determining factor is the contrast 

 factor of the photographic plate. Plates of high contrast will 

 give maximum sharpness, and since 7 is low in the violet but 

 increases with the wave-length, isochromatic plates used with 

 a yellow filter will give increased sharpness and resolving 

 power. Yellow-dyed plates, on the other hand, are not 

 suitable for astronomical work ; they have low turbidity, the 

 advantage of which is lost on account of the optical turbidity, 

 and they have low contrast, resulting in poor resolving power 

 and sharpness. 



Ross queries whether astronomers are right in asking for a 

 high-speed plate of fine grain. He states that the resolving 

 power of modern plates is far ahead of that which can be 

 obtained with the largest telescopes. An increase in speed of the 

 plates will increase the resolution obtained with a large telescope ; 

 but it is doubtful whether an increase in the resolving power 

 of the plate will do so. Provided that increased speed can be 

 obtained without loss of the resolving power which the plates 

 now have, he claims that it is more profitable for astronomical 

 photography to work in the direction of attaining higher speed 

 than attaining better resolving power. 



" The Mutual Action of Adjacent Photographic Images " 

 is dealt with in a fourth paper {ibid., 53, 349, 1921). The 



