Rood — Photometric Method independent of Color. 173 



Aet. XXIY. — On a Photometric Method which is indepen- 

 dent of Color ; by Ogden K Rood. 



The principle underlying most of the photometric methods 

 now in use depends on a comparison of the illumination of two 

 adjacent fields ; in some instances as where a spot or a ring is 

 made to disappear, this idea is thinly disguised, but instantly 

 becomes apparent when the lights are differently colored. 



The object of the present paper is to present a mode of pro- 

 cedure in which the coloration of the two surfaces, even 

 though at a maximum of vividness, is a matter of entire indif- 

 ference, since the process depends not on a comparison of 

 these surfaces, but on the shock which the retina experiences 

 when one surface is quickly withdrawn and replaced by another. 

 If we take a uniform circular disc of any color, illuminate it 

 evenly, and then set it in rotation slowly or rapidly, the retina 

 will receive no shock, since the parts replacing each other in 

 the retinal image are in all respects identical, in other words 

 no nickering will be perceived. If, however, one-half of the 

 disc reflects less light than the other by fa of the total amount, 

 with appropriate rates of rotation a faint flickering will be 

 noticed, which will increase in intensity as the difference of 

 the luminosities of the two halves of the disc is made greater. 



Accordingly I prepared a set of about a hundred discs of 

 drawing paper, their depth of tint ranging from the whitest 

 white paper down to the blackest, the gradation being as even 

 -as practical. For the determination then of the reflecting 

 power, for example of a vermilion disc, it was only necessary 

 to select from the series a gray disc which when combined 

 with it in equal parts gave no perceptible flicker, and after- 

 wards to determine the reflecting power of this gray disc in 

 terms of the standard white cardboard. Results were thus 

 obtained for the principal colors, red and blue-green, yellow 

 and blue, green and purple. The difficulty in measuring the 

 reflecting power of these colored discs was in all cases the 

 same, and was due to the fact that it is hard to obtain gray 

 discs with absolutely uniform surfaces. This trouble, or a lack 

 of uniformity in the colored discs themselves, was to a con- 

 siderable extent obviated by reversing the halves of the com- 

 posite disc so as to employ the previously unused surfaces. 

 Small irregularities, texture, or a sandy appearance do not 

 interfere with the use of the discs to a noticeable extent, but 

 when the depth of the tint changes slowly over an extended sur- 

 face it gives more trouble. Of course the case frequently arose 

 where in the entire hundred discs not one could be found with 



