Gradation of a Developed Photographic Image. 307 



its coefficient of absorption (obstruction). The mode of measurement 

 has been described in other papers by myself and need not be repeated. 

 As the " star" opacities and the graduated opacity scale on each plate 

 were measured with the same annulus, it was unnecessary to reduce 

 the measurements to densities which are usually taken in terms of 

 common logarithms, or to transparencies in percentages of the initial 

 light. 



Example of Experiments. 



It will facilitate matters if one example of measures be given in 

 detail, and the mode in which they are applied. The spectrum colour 

 used was at the scale No. 56*7. The star with the plate in contact 

 with it was placed in the dark slide, and so arranged that the square 

 patch of monochromatic red light would cover the whole of the former. 

 The only light which would penetrate to the plate was through the 

 star opacities. The star and plate were made to revolve round their 

 centre in the slide by means of a spindle projecting outside, on which 

 was a pulley that could be geared to an electromotor. Exposure was 

 given for 65 minutes. No light was in the room except the red light. 

 To make certain that the red light which fell on the prisms, and which 

 illuminated them to a certain small extent, had no effect on the plate, 

 the slit S, fig. 2, was covered with red glass, which only allowed the 

 red of the spectrum to pass. The plate after the first exposure was 

 completed ; was removed and placed in a special slide, which allowed 

 varying time exposures to be made on small square areas of the plate 

 alongside that part which had been already impressed. The exposures 

 were made to an amyl-acetate lamp at 4 feet distance, and were of 1, 

 2, 4, 8, &c, units of time duration. The plate was developed with 

 ortol developer, fixed, washed, and dried. It was then placed in the 

 measuring apparatus, and the scale densities of the amyl-acetate lamp 

 exposures and the star opacities measured. On looking at Table I it 

 will be seen that the coefficient of absorption, as there shown, is 0'87. 

 The numbers in Table II were therefore multiplied by 0*87 to give 

 the scale for abscissa in powers of 2. The following measures were 

 obtained (Tables IV and V). 



These results were plotted (fig. 4), and straight parts of both curves 

 were compared. It will be seen that in the star opacities the curve 

 cuts the abscissa 1 with an ordinate of 174, and this same ordinate is 

 found on the scale curve at 2*65 in the abscissa. Again, the first has 

 an ordinate of 63 at the abscissa 4, but the scale has abscissa 6'65 for 

 the same ordinate. This shows that the exposures of the star would 

 have had to be prolonged in both cases to have acquired the same 

 density as the scale, but very unequally. We can find the unequal 

 times necessary by subtracting the two abscissae from one another at 

 each point, and expressing the inequality by a fraction, 



