1893.] On the Failure of a Laiv in Photography. 145 



The experiment which naturally suggested itself was to expose a 

 sensitive surface to the action of the light of an amyl acetate lamp 

 passing through a slit as before, the drum on which it was stretched 

 being caused to rotate at high and low speeds, and also to place on 

 the same paper a scale of exposures with the drum at rest. The?e 

 were all developed together. An example of one of many experi- 

 ments is given as an illustration. 



The circumference of the drum with the paper stretched round 

 it was 12'25 in. The width of the slit was arranged to be 0'012 in. 

 The amyl acetate lamp was placed 2 ft. from the slit, and a rotation 

 of 30 per sec. was given to the drum for one exposure and 1 per sec. 

 for a second exposure. In the first case the time of exposure during 



each revolution was 7 X -- sec., or about 1/30,000 sec. 

 12'^5 oO 



The sum of the exposures during 20 min. was thus 1'176 sec. 

 In the other case the expos are was 



?, or about 1/1000 sec, 



and the sum of the exposures was, as before, 1*176 sec. Thus the first 

 individual exposures had only ^ ^ ^he duration of the second 

 exposures, though in the aggregate they were the same. 



A scale of blackness was made on the same paper, through a square 

 aperture, without shifting the lamp, the exposures being J, J, -jr, 1, 2, 

 4, and 8 sec. On developing it was apparent to the eye that the 

 first band was much lighter than the second. The scale and blackness 

 of the bands were measured accurately, and the times of exposure 

 which had been given to each band, on the assumption that the law 

 enunciated held good, were calculated and found to be for the first 

 band 0'6 sec., and for the second band 0'91 sec., instead of 1'176 sec. 

 which was really given in all. Another example is where the slit 

 was opened to O'll in, and the time of exposure reduced from 20 to 

 10 min. It was found that in this case the exposures given on the 

 same assumption were 3'7 sec. and 5'28 sec, the real exposure given 

 being 5*36 sec. The last experiment shows that if the slit had been 

 slightly wider or the rotation slower the law would have been 

 approximately obeyed. 



Another experiment was made by throwing an image of the crater 

 of the positive pole of the electric light on a hole bored in a plate 

 about 2^ in. in diameter by means of a lens, and allowing tne emergent 

 beam to fall on the slit and paper, the drum being made to rotate as 

 before. The same kind of results were obtained. 



As it might be thought that this difference was caused by some 

 action other than chemical, another series of experiments was under- 

 taken. In these different sensitive surfaces were employed in order 



YOL. L1V. L 



