316 PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



(b) Intensity and Time for Equal Blackness. Experiments have proved that for 

 the same photographic plate and development the intensities of light and the 

 durations of exposure necessary to produce the same blackness on the film bear a 

 certain relation to each other. According to MICHALKE this relation is independent 

 of the degree of blackness. I have represented MICHALKE'S observations by the 

 formula 1 1" = constant, where a= T08, and further redetermined a for the plates 

 employed by me (Imperial special rapid). I used as a source of light a disc of opal 

 glass (2 inches in diameter), illuminated from behind by an electric lamp, and I 

 exposed directly to the light of the disc successively different portions of the same 

 plate at distances varying from 1 to 15 metres. A Thornton-Pickard shutter 

 recorded the duration of exposure automatically on a chronograph. My value of a is 

 1-05 0-01. 



(c) The ratio of two intensities, i m , i n , which illuminate, through apertures a m , a ot 

 a lens during times < M , t n , a photographic plate placed in the focus of the same camera 

 is a constant if they produce equal blackness on the film. By (a) and (b) 



i~ n = a m \tj 



F mH can be calculated for the eclipse photographs. The individual exposures of 

 Photographs VI. and VII. are uncertain to about a second (see 6), but their sum is 

 accurately known (100'88). I take here t 6 = ll'OO, and hence t- t = 89'88. The other 

 data are given in 1. The numerical value of a is of no importance for the first five 

 photographs. 



m, n 1,2 1, 3 1, 4 1, 5 5, 6 6, 7. 



log F 0-278 0-551 0-884 T335 1'059 0'869. 



(d) Correlative Distances on the Corona. If the pictures of the corona had not 

 been overlapping, and the sky been dark, an equal-blackness curve would have 

 coincided with an equal-intensity curve of the corona, and the ratio of the intensities 

 of the corona belonging to two such curves on Photographs m and n would equal a 

 constant F mB [see (c)]. I shall call "correlative distances on corona" the distances of 

 points of the corona at which the ratio of the intensities equals F mn . 



(e) Simultaneous and Successive Exposures. I make the following two assump- 

 tions : (1) the degree of blackness on the film is independent of the order in which 

 two or more exposures are made ; (2) if two intensities give the same blackness for 

 certain exposures, they do so, too, when these exposures are made on an otherwise 

 exposed film. I have checked (1), but not (2), by experiment. 



Let two intensities I and S illuminate the film together during the same time t. 

 By (b) 



(I + S)" = S'(t + t') = S a * + S"f' for equal blackness, 



