Gradation of a Developed Photographic Image. 313 



Experiments with Fixed Intensities of Rays. 



Before commenting on this curve it will be better to describe the 

 next set of experiments in which the light is constant, and there is a 

 change in time. 



The arrangements made were as follows :— Four slits in a card 

 were made of such convenient width as (found by trial) allowed four 

 different rays of the spectrum to emerge, and in front of the slits were 

 cemented strips of a spectacle lens, which each gave an image of the 

 prism surface of small size, but alongside one another. To prevent 

 the white light which illuminated the prisms causing any error in the 

 exposure, in front of each sl^fc was placed a strip of glass of a colour 

 approximately corresponding to the colour coming through it. 

 Exposures were made to the four colours in the same plate and for the 

 same length of time, the exposure being admitted or shut off at the 

 slit of the spectroscope, and when completed the plate was given a 

 graduated scale with the amyl-acetate lamp as before. The develop- 

 ment of the plate was then carried out and the densities measured as 

 usual. 



The curve of the amyl-acetate light was plotted first, and the places 

 which corresponded to the density of the " blue " light scale was 

 marked on it. It was necessary to do this, for although the electric 

 arc light was steady, yet it did not remain absolutely the same in 

 intensity throughout the whole of the exposures. The places so fixed 

 on the scale made by the amyl-acetate lamp by the blue exposures 

 gave the points in the abscissa to which to refer the ordinates of the 

 three other colour curves. These were duly set up and the curves 

 drawn. Fig. 6 shows Table IX drawn diagrammatically. It was 

 again found that the gradation given by the colours less refrangible 

 than the Scale No. 24 were steeper than that of this No., as were also 

 those of the colours more refrangible. 



The slits were then moved into new positions and the same process 

 gone through. (See Tables IX, X, and XL) When these gradation 

 factors are plotted on their appropriate scale numbers we get a curve 

 convex to the base, with the lowest part lying about Scale No 24, con- 

 firming the results obtained by the previous place. (See fig. 5.) There 

 can be but little doubt from both of these results that the place of 

 minimum gradation given by rays is close to the wave-length to which 

 the salt of silver under consideration is most sensitive. 



