1897.] NEW YORK ACADEMY OF SCIENCES. 197 



fessor Rood for the purpose of studying more elaborately the 

 phenomena of flickering and finding out, if possible, whether or 

 not it is entirely independent of differences in color. 



Before the phenomenon of flickering is extensively used in 

 photometric work it is necessary that tlie phenomenon should 

 be elaborately studied with a view of distinguishing it from all 

 allied phenomena with which it is at all likely to be confounded. 

 It is also necessary that the effect, if there is any, of difference 

 in color upon the phenomenon of flickering should be subjected 

 to as rigorous experimental investigation as possible. And 

 finally, the advantages of having results verified by more than a 

 single pair of eyes should not be lost sight of. 



For the study of flicker phenomena there is perhaps no more 

 convenient means than that afforded by a graduated series of 

 gray disks and, as the experiments of Professor Rood had, so far 

 as I knew, never been verified by another pair of eyes, a series 

 of gray disks similar to the series used by Professor Rood was 

 prepared and used throughout the following investigation. 



The Apparatus Used. 



Two graduated sets of gray disks were used, a large set five 

 and one-half inches in diameter, and a small set three inches in 

 diameter. The disks of the large set were numbered 7, 8, 0,. . . . 

 etc., from the lightest to the darkest shades. The set included 

 one hundred and twenty-nine disks. The disks of the small set 

 were similarly numbered 7, 8, 9,. . . .etc., and contained the same 

 number of disks. The luminosities of these disks were deter- 

 mined in the ordinary way by the use of a black and a white disk 

 and are given as so many per cent, of the luminosity of white 

 bristol board which is taken as one hundred per cent. The de- 

 termined luminosity of the lightest disk of either set was 89 per 

 cent., of the darkest disk, 4 per cent. Whenever in the succeed- 

 ing portions of this article the difference in luminosit}^ between 

 two disks, gray or colored, is given as so many per cent., by that 

 is meant a difference of so many per cent, of the luminosity of 

 white bristol board. 



Two sets of colored disks of the same sizes as the gray disks 

 were kindly furnished me by Professor Rood. As the correspond- 

 ing large and small colored disks were cut from the same piece of 

 paper, they are of the same tint and luminosity. 



Each disk was slit radially and perforated in the center (Fig. 

 I. A. and B.). They could thus be combined pair-wise (Fig. J. 

 C) and placed on the axis of the rotation apparatus. The rota- 

 tion machine was placed at a distance of about one and a-half 

 meters from a window of southern exposure. The disks, when 



