204 TRANSACTIONS OF THE [MAR. 1, 



small gray disk is less luminous if the above assumption is cor- 

 rect. Therefore the luminosity of the colored disk is found by 

 taking the mean of the luminosities of the large and small gray 

 disks. 



The application of this method to the determination of the 

 luminosity of the yellow disk used in the previous method gave 

 the following results. After a few trials it was found that the 

 yellow disk, when combined pair-wise with the large gray disk 

 No. 51, and the small gray disk No. 30 (see Fig. 2, A and B) 

 and rotated, gave outside and inside flickers of equal violences. 



The measured luminosity of the large gray disk No. 51 was 48.0 per cent. 



The measured luminosity of the small gray disk No. 30 was — 55.5 per cent. 



2)103.5 



The luminosity of the yellow disk is, therefore, according to 



this method 51.7per cent. 



The average of seven similar determinations gave 50.4 per cent, 

 as the luminosity of the yellow disk. The luminosity obtained 

 b3^ the method of Least Flicker was 50.9 per cent. The maxi- 

 mum difference between the mean of the seven determinations 

 by the method of Equal Flickers and any one of them was about 

 4.0 per cent, of the mean. 



A comparison of a large number of determinations, by the 

 method of Equal Flickers of the luminosity of a given colored 

 disk, in which the difference in luminosit}' between the large 

 and small gray disks used varied from about 5 per cent, to about 

 85 per cent., showed that so long as this difference was less than 

 20 per cent., the luminosities obtained for the colored disk agreed 

 with each other and with the luminosity obtained by the method 

 of Least Flickers. For greater differences than about 30 per 

 cent., the results did not show so good an agreement. So long, 

 however, as the difference was kept less than 20 per cent., it was 

 found that the figures obtained by the method of Equal Flickers 

 for the luminosities of various colored disks agreed with those 

 obtained by the method of Least Flicker, as is shown in the fol- 

 lowing table : 



Description of the Luminosity of colored disks as determined by the 



colored disks. Method of Least Flicker. Method of Equal Flickerii 



Bright Red 16.1% 16.3% 



Dark Blue 6.6% 6.5% 



Green 46.5% 46.5% 



Bright Yellow 50.1% 50.4% 



Light Orange 55.0% 54.6% 



Each of the luminosities given under the method of Least 

 Flicker is the average of not less than two determinations and 



