1807.] NEAV YORK ACADEMY OF SCIENCES. 203 



such a position as to exclude from view all of the disk except the 

 portion inclosed by the rectangle in Fig. II. B. 



The following is an example of the application of the method. 

 It being desired to find the liiminosit}^ of a yellow disk, various 

 large gray disks were successively combined pair-wise with it 

 and rotated, and the large gray disk No. 40 was found to give 

 only a slight flicker when rotated with the 3-ellow disk. The 

 small gray disks Nos. 44, 43, 39 and 31 respectively gave less 

 violent flickers with the colored disk than the flicker given by the 

 large gray disk. No. 40, and the colored disk. Of the small 

 gray disks. No. 43 seemed to give the least flickering with the 

 colored disk. Only one large gray disk, No. 39, was found to 

 give as little flicker with the colored disk as the small gray disk, 

 No. 43, had given. 



The measured luminosity of the large gray disk No. 39 was 51.9 per cent. 



The measured lumiuosity of the small gray disk No. 34 was 50.0 per cent. 



2)101.9 



The luminosity of the j'ellow disk is therefore ,50.9 per cent. 



Three other determinations of the luminosity of the same yel- 

 low disk made in a similar wa}^ gave for its luminosity 49.5 per 

 cent., 50.0 per cent, and 50.0 per cent, respectively. The mean 

 of the four determinations is 50.1 per cent. 



The second method used in the determination of the luminosi- 

 ties of coloi'ed disks, which was devised by myself, I will call 

 the method of Equal Flickers. The validity of the method de- 

 pends upon the assumption that the flicker produced by the ro- 

 tation of a pair-wise combination of a gray and colored disk in 

 which the gray disk is, to a moderate extent, more luminous than 

 the colored disk, is of the same violence as the flicker produced 

 by the rotation of a pair-wise combination of the same colored 

 disk with a gray disk less luminous than the colored disk b\^ the 

 same amount. 



In the practice of the method a large gray disk is selected 

 which is somewhat more luminous tlian the colored disk. This 

 is combined pair-wise with the colored disk, and various small 

 gray disks somewhat less luminous than the colored disk are 

 successively combined with the large gray and colored disks, as 

 in Fig II. B, until a combination is found which on rotation 

 gives outside and inside flickers of equal violence. The violence 

 of the two flickers is compared in the way already described. 

 When the two flickers are of the same violence the large gray disk 

 should be as much more luminous than the colored disk as the 



