1897.] NEW YORK ACADEMY OF SCIENCES. 205 



each of the luminosities given under the method of Equal 

 Flickers is the average of not less than five determinations. 

 The difference between the luminosities of a given color deter- 

 mined by the two methods is in no case greater than 0.4 per 

 cent. The advantage of the method of Equal Flickers over that 

 of Least Flicker is that it enables one to work with a smaller 

 number of gray disks. 



Since the preparation of a graduated series of gray disks in- 

 volves considerable labor, every one wishing to carry on experi- 

 ments in flicker photometry may not care to prepare them. Pro- 

 fessor Rood in his article in the American Journal of Science for 

 September, 1893, suggested that the luminosity of a given colored 

 surface might be determined by matching the color using only a 

 few standard disks, the luminosities of which had been previously 

 carefully determined by someone possessing a graduated series 

 of gray disks. In applying this method to the determination of 

 the luminosities of colored surfaces I selected for my standard 

 disks the foUowins:. 



Experiments to test the applicability of the method were made 

 upon the following four disks : a straw yellow, (S. Y. ), a light 

 orange, ( 0. ), a dark brown, (Br.), and a very dark green, 

 ( D. G. ). Each of the following four color equations is the 

 mean of ten determinations : 



100 per cent. S. Y. = 15.6 per cent. G. + 30.4 per cent. Y. +11.3 

 per cent. R. + per 9.3 cent. W. + 33.4 per cent. Bk. 

 The luminosity of the straw yellow disk calculated from this 

 equation is 34.3 per cent. 



82.2 per cent. 0. + 17.8 per cent. Bk. = 40.5 per cent. R. + 39.1 

 per cent. Y. + 20.4 per cent. W. 

 The luminosit}^ of the light orange disk calculated from this 

 equation is 56.6 per cent. 



100 per cent. Br. = 18.0 per cent. R. + 27.0 per cent. Y. + 3.9 per 

 cent. W. + 51.1 per cent. I3k. 

 From which we obtain for the luminosity of the dark brown 

 disk 23.0 per cent. 



