174: Rood — Photometric Method independent of Color. 



which the flickering entirely disappeared with quite low rates 

 of rotation, and here it was necessary to determine with which 

 gray disc the flicker was at a minimum. This was accom- 

 plished by combining the same colored disc with two gray 

 discs, a larger and a smaller, on the same axis, when it would 

 become evident which was the more favorable combination, 

 and the observer would notice that the luminosity of the col- 

 ored disc must be nearer to one gray disc than to its mate, or 

 about equally distant from both. 



As before stated results were obtained for six discs, but 

 these were selected so as to be complementary to each other in 

 pairs, and in order to test the process they were now combined 

 pair-wise and the resultant luminosities of their gray mixtures 

 were determined by the old method, and afterwards calculated 

 on the basis of the figures furnished by the nickering process. 



Gray mixture. Diff. Gray mixture. Drff. 



-r, i j \ 2 1 -5 observed A „ , , ,, ( 20*2 observed Q 



Purple aud green j ^ ca]culated Red and blue-green } ^ calculated T 9 o 



Gray mixture. DifE. 



Tr n ^ ui { 27*85 observed , or; 



Yellow and blue | 29 . lcalculated 125 



These experiments were not at all elaborate, and as their 

 greatest difference barely exceeds one per cent of the reflect- 

 ing power of white cardboard, they may be taken as furnish- 

 ing a proof of the correctness of the process employed. 



Thus far we have dealt with the combination of white 

 (gray) discs with those that are strongly colored, and it remains 

 to give an example of the process as applied to two differently 

 colored but not complementary discs. To test this matter it 

 was necessary to find two colored discs having the same or 

 nearly the same reflecting power. In my collection I finally 

 found two such discs, a cyan-blue with a reflecting power of 

 23*9 and a purple for which the figure 23*3 had been obtained : 

 these discs when combined gave a scarcely perceptible flicker. 

 Since then, graded series of yellow discs have been made, but 

 it has been impossible to find time to hunt up their equivalents 

 in luminosity and make the necessary determinations. 



This flickering process having answered so well, the proce- 

 dure was reversed, and used with great advantage to facilitate 

 the determinations of the values of the gray discs executed in 

 the ordinary way ; in other words the series of gray discs as 

 made by myself is not pure gray but has a slightly yellowish 

 tint that makes estimation on equality of luminosity a little 

 more difficult than it ought to be. Accordingly in measuring 



