316 Capt. W. de W. Abney. Photographic Values' [Mar. 19. 



time and altering the intensity of the light acting. Thus we may 

 place an amyl acetate lamp or a candle JL6 feet away from a sensi- 

 tive surface, and expose different small portions of the plate for 2, 4, 

 8, 16, &c, seconds; or we may place the amyl acetate lamp at 1, 2, 

 4, 8, &c, feet away from the plate, and, after each move, expose a 

 different portion of the plate for, say, 20 seconds. Up till recently 

 it has been held that the resulting chemical action is the same in 

 both instances, so long as the " time " X " intensity " is the same, 

 so that an exposure to a light 10 feet away from the plate for 

 10 seconds effects the same chemical decomposition of the sensitive 

 salt as an exposure of 1000 seconds when the lamp is placed 

 100 feet away. This I have shown in the ' Proceedings of the 

 Royal Society,' and in the 1 Photographic ' and ' Camera Club ' 

 Journals, to be wide of the mark if the sensitive surfaces are what 

 we may call slow, though it is practically the case when using rapid 

 plates. (It should be remarked that for printing-out processes this 

 variation has not so far been found.) This was a point to which my 

 attention was necessarily directed, and the readiest means of proving 

 if the plates to be employed were suitable, was not only to expose 

 through the scale of opacities, but subsequently to impress on the 

 same plate a scale obtained by exposures to a fixed source of light 

 but with varying times. If when plotted the two curves were identical 

 or parallel, the proof was sufficient to show that the plates might be 

 safely employed without any error creeping in. For it may be 

 remarked that the straight part in what we may call the " intensity " 

 curve is, in a slow plate, always less steep than in the time curve. 

 With the plates that were used the " time " and " intensity " curves 

 were found to fulfil the condition of parallelism. 



Fig. 1. 



