208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



taking of motion pictures, are given exposures of the order of one- 

 jBftieth of a second. Most astronomical photographs are taken with 

 exposures of the order of an hour. There is thus a factor of about 

 10^ between the two times of exposure. 



W. deW. Abney first observed that photographic materials do not 

 adhere to the reciprocity law of Bunsen and Roscoe; that is, if the 

 brightness of the exposing light is diminished 100 times and a plate 

 is exposed to it 100 times as long, exactly the same result is not ob- 

 tained. The relation between the exposure and the photographic 

 effect was measured for the range of astronomical levels by K. 

 Schwarzschild in 1899, and he came to the conclusion that a constant 

 effect is produced as long as the condition /, the intensity, multiplied 

 by the time to a constant power is maintained constant; that is, ef- 

 fective exposure =/ii^. For the materials he was using, Schwarzs- 

 child found the constant p to be about 0.8. It is now known that this 

 is not a valid criterion for determining a constant photographic effect 

 over very wide ranges of intensity. It holds fairly well over a limited 

 range, and in astronomical photography the range is generally limited. 

 But the relation between exposure and intensity is actually a curve of 

 a catenary shape, and photographic emulsions can be made to have 

 their maximum sensitivity at different levels of intensity. The plates 

 which for so many years were standard in astronomical photography 

 were those made primarily for use in portraiture, and such plates are 

 made to have their maximum sensitivity under normal photographic 

 working conditions; that is, for exposures of a fraction of a second. 

 If, however, the emulsions are modified in manufacture, it is possible 

 to increase the sensitivity appreciably at low intensity levels though 

 a loss of sensitivity may be incurred for short exposures. For in- 

 stance, two plates used today by astronomers are known as I-O and 

 103a~O. If we tested the speed of these two plates by practical ex- 

 posure, we should find that the 1-0 plate is nearly twice as fast as the 

 103a-O when used by a press photographer ; when used by an astron- 

 omer, the 103a-O plate would be about three times faster than the I-O. 



The realization of the importance of this reciprocity failure led 

 us to make special emulsions (of which 103a-O is an example) in 

 which the reciprocity failure at low intensities was reduced to an 

 absolute minimum and the plates were made essentially to be used 

 for exposures of the order of hours. So successful has this experi- 

 mental work been that I was told some years ago by W. S. Adams 

 that our plates had made the 100-inch telescope at Mount Wilson 

 effectively as useful in regard to exposure time as they had expected 

 the 200-inch at Mount Palomar to be when it was completed. This 

 does not mean, however, that the value of the 200-inch instrument is 

 in any way less. 



