1897.] new york academy op sciences. 201 



The Measurement of the Luminosities op Colored Disks. 



In this section the methods employed in measuring the lumi- 

 nosities of colored disks will be described. The first method 

 which I have called the method of Least Flicker is essentially the 

 same as was described by Professor Rood in the American Jour- 

 nal of Science for September, 1893. The validity of the method 

 depends on the assumption that the violence of the flicker, 

 •observed when two disks are combined pair-wise and rotated 

 before the eyes, decreases if the difference between the luminosi- 

 ties of the two disks thus combined is diminished, no matter 

 what the difference in color may be, and that for zero difference 

 in luminosity all flicker vanishes. 



In applying this method to the determination of the luminosity 

 of a given colored disk various gray disks are successively com- 

 bined pair-wise with the colored disk until one is found which on 

 rotation gives no flicker. The luminosit}' of the gray disk can 

 then be determined in the ordinarj^ way and this will be the 

 luminosity of the colored disk if the above assumption is cor- 

 rect. In the practice of the method two difficulties will in 

 general be encountered. 



First : As Professor Rood has pointed out, the entire series of 

 gray disks may not contain one of exactly the same luminosity 

 as the colored disk and we will on this account be unable to get 

 rid of the last trace of the flicker. 



Second : Unless the gray disks are very carefully prepared, 

 slight differences in the luminosities of different parts of the 

 same disk ma}^ give rise to a greater flicker than that due to the 

 difference in luminosity between the gray and the colored disk. 



On account of these two difficulties I was in general unable to 

 find a gray disk which would give absolutel}' no flicker when 

 rotated in a pair-wise combination with a colored disk and had, 

 therefore, to be contented with the gray disk which gave the 

 least flicker. The selection of the gray disk giving the least 

 flicker with the colored disk was made as follows : 



A large gray disk giving only a slight flicker, when combined 

 pair-wise with the colored disk and rotated, was selected. The 

 flicker thus produced was used as a standard for comparison and 

 all the small gray disks giving a less violent flicker, when rotated 

 in a pair-wise combination with the colored disk, were selected. 

 From these small gray disks the one judged to give the least 

 violent flicker with the colored disk was chosen, and this flicker 

 used as a second standard for comparison in the selection of all 

 large gra}' disks giving a less violent flicker with the colored 

 disk. From the large gray disks thus selected the one judged 



