146 On the Failure of a Law in Photography. [June 1] 



to eliminate any possibility of the effect being due to any phosphoi 

 escence of the paper, though none could be detected. Plates 

 held stationary and exposures made by admitting light to portioi 

 of them through slits of known angular aperture, cut in a disc whi( 

 could be rotated at any desired speed. Similar results were obtaiiK 

 to those already described. The quickest rotation gave the Je? 

 density. It may be remarked that the more sensitive a surface is 

 radiation the less marked are the differences observable for the 

 speeds of rotation. This is what might be expected. 



As an outcome of the experiments so far made, it seems that wh< 

 exposures less than 1/1000 sec. are made on a sensitive surface, ai 

 the source of illumination is an amyl acetate lamp (Von Altneck'g 

 placed 1 ft. from the sensitive surface, the law quoted ante fails. 



The question of a very low intensity of light acting and of tl 

 sensitiveness to different spectrum colours is now occupying 

 attention. 



Addendum. July 4, 1893. 



Since the above paper was read I have made an investigation into 

 the question as to whether the foregoing law fails when feeble inten- 

 sities of light are acting, and find that it does so signally. Sensitive 

 surfaces were exposed in a Spurge sensitometer, in which there are 

 thirty graduations of light admitted to different parts of the surface 

 at the same time, the intensity of light being varied by its admission 

 through apertures of varying size. The smallest aperture used was 

 1/256 of the largest, and an exposure lasting 2650 sec. was given 

 to the former, whilst 10 sec. was given to the latter. It was 

 found that the blackness produced by the two was very different, 

 that produced by the light passing through the small aperture corre- 

 sponding to an area of 1/600 of that passing through the largest 

 aperture, if the law held good. The light employed was a large 

 illuminated surface, which was equal to one amyl acetate lamp placed 

 6^ in. away from the surface, without passing through the sensito- 

 meter. 



As some persons might doubt the accuracy of this method, a 

 different mode of experimenting was adopted in the next series made. 

 An amyl acetate lamp was used, and portions of a sensitive surface 

 were exposed at different distances from it, on the assumption that 

 the squares of the distances gave a measure of the exposures neces- 

 sary in order to produce equality of chemical action. In one experi- 

 ment exposures were made at distances of 2, 4. 8, 12, 16, 20, and 

 24 ft. ; the duration of exposure at 2 ft. being 10 sec., whilst for the 

 last it was 24 min. The intermediate exposures were calculated 

 on the same principle, a scale of blackness was also made by exposing 



