ASTRONOMICAL PHOTOGRAPHY — MEES 207 



sensitivity in their photographic materials. The news photographer, 

 for instance, wants the highest speed that can be obtained, so that he 

 can photograph at night with a minimum of flashlight. The motion- 

 picture photographer wants to take pictures with a minimum of light. 

 The aerial photographers, and especially those taking military pic- 

 tures, want to give very short exposures and to work when the light 

 is poor. Improvements in the quality of photographic materials 

 are therefore valuable to all classes of photographer, but, as shown 

 later, the astronomer has some additional requirements. 



There is a relation between the sensitivity and the size of the grains 

 of an emulsion since the larger the grains, the more effective the ex- 

 posure of a grain in producing silver on development. It must be 

 remembered that the unit of exposure is the silver-bromide grain. 

 The most sensitive grains of a fast emulsion become developable 

 when they absorb a few quanta of light. The sensitivity that can 

 be obtained with grains that are not spontaneously developable with- 

 out exposure seems to reach a limit, with a requirement of approxi- 

 mately 10 quanta per grain. Once exposed, the grains are completely 

 developable. 



The amount of silver produced by exposure is very small, but de- 

 velopment produces at least a million times as much silver as is present 

 in the exposed grain. The larger the grain, the more silver produced 

 and the greater the multiplying factor introduced by development. 

 Unfortunately, the use of larger grains, which give increased sensi- 

 tivity, produces a granular appearance in the image and a limitation 

 of sharpness and resolving power. The astronomer has always re- 

 quired, continues to require, and will always require an increase of 

 sensitivity with no increase in granularity, and he has been telling me 

 so for the last 40 years ! It is even difficult to get him to answer the 

 question that we put to him : "Would you rather have fineness of grain 

 or sensitivity ?" since his invariable answer is that he wants both. In 

 practice, the choice between the two is one which has to be made by 

 the astronomer, gently guided, let us say, by the manufacturer of 

 the plates. Sometimes the same program, or what appears to a lay- 

 man to be the same program of tAvo different observatories, in one case 

 demands plates having maximum sensitivity with whatever graininess 

 is necessary to achieve it, while the other requires a moderate sensitiv- 

 ity accompanied by a decreased graininess and enhanced resolving 

 power in the images. 



As we continued to study the properties of photographic materials, 

 we realized that there is a fundamental difference between the con- 

 ditions under which most astronomical photographs are taken and 

 those in which the photographic materials are exposed in other fields 

 of photography. This is in the time of exposure. The greater num- 

 ber of ordinary photographic negatives, including those used for the 



