238 



Colour Photometry. 



[May 31, 



then equalised by the sectors and the relative intensities of the two 

 reflected rays calculated. This was repeated throughout the spec- 

 trum. Vermilion, emerald- green, and French ultramarine were first 

 measured by the above method and then sectors of these colours 

 prepared, which when rotated gave a grey matching a grey obtained 

 by rotation of black and white. The luminosity curves of these three 

 colours were then calculated and reduced proportionally to the angle 

 that each sector occupied in the disk. The luminosity curve of the 

 white was then reduced in a similar manner, and it was found that the 

 sum of the luminosities of the three colours almost exactly equalled 

 that of the white. The same measurements were gone through with 

 pale-yellow chrome and a French blue, which formed a grey on rota- 

 tion, with like results. It was further found that the sum of the in- 

 tensities of vermilion, blue, and green varied at different parts of the 

 spectrum, and the line joining them was not parallel to the straight 

 line which represented white for all colours of the spectrum and 

 which, itself was parallel to the base. Since a straight line parallel 

 to the base indicated degraded white, it followed that if the intensity 

 of the rays of the spectrum were reduced proportionally to the height 

 of the ordinates above a line tangential to the curved line (which 

 represented the sum of the intensities of the three colours at the 

 different parts of the spectrum) and were recombined, a grey should 

 result. A method was devised of trying this, and the experiment 

 proved that such was the case. The same plan enabled the colour of 

 any pigment to be reproduced from the spectrum on the screen. 

 The combination of colours to form a grey on rotation by a colour- 

 blind person was also tried, and after the curve of luminosity of the 

 colour's had been calculated and reduced according to the amount 

 required in the disk, it was found that the sum of the areas of the 

 curves was approximately equal to the white necessary to be added 

 to a black disk to form a grey of equal intensity as perceived by him. 

 The spectrum intensity of gaslight in comparison with the electric 

 light was also measured, and the amount of the different colours 

 necessary to form a grey in this light was ascertained by experi- 

 ment. 



As before, it was found that the calculated luminosity of the colours 

 was equal to the white which combined with black formed a grey of 

 equal luminosity. 



The question of the coloured light reflected from different metals 

 was next considered, and the method of measuring it devised, as was 

 also the method of measuring absorption spectra. The luminosity 

 curves obtained by the old method were compared with those ob- 

 tained by the present method, and so close an agreement between them 

 was found to exist, as to give a further confirmation that our former 

 plan was accurate. A number of pigments that can be used for 



