Rood — Photometric Method independent of Color. 175 



the reflecting power of the gray discs by the old method, the 

 standard white and blacks discs were combined with a small 

 amount of an orange-yellow disc which made their mixture 

 match the tint of the gray disc under experiment. The lumi- 

 nosity of the orange-yellow disc in terms of standard white 

 card-board being known, the final result was calculated with 

 equal facility and the annoyance removed. It is hardly neces- 

 sary to add that in all cases the amount of white light reflected 

 from the standard black disc was taken into account. 



An experiment was now made to test the flicker process by 

 using it to determine the reflecting power of two gray discs 

 which were afterwards measured in the ordinary way, and the 

 following result was obtained : 



Direct determination . 34*1 29*1 



Flicker " 33-7 28'6 



Difference *4 *5 



These determinations were not elaborate but may suffice for 

 the purpose. 



Afterwards a more careful set of observations was made to 

 ascertain with what minimum difference in luminosity the 

 flicker became insensible : a series of five light-gray discs was 

 made with gradations so small that these would have required 

 elaborate work for their individual determination which was 

 not attempted, directly, the method of measuring the least 

 and the most luminous disc being employed, and the difference 

 between the figures divided by four. When these discs were 

 tested flicker fashion in pairs, it was found that the intervals 

 between them were not at all identical, one being larger and 

 its neighbor smaller than the average interval, but each pair 

 nevertheless furnished a faint but distinct flicker. If the 

 intervals had been regular, as was intended, the experiments 

 would have proved that with a difference of -^ part of the 

 total light a flicker could be perceived ; as it was, it showed that 

 a considerably smaller difference suffices, probably about T ^. 



It is evident that for the purpose of measuring the reflect- 

 ing power of colored discs, it will not in every case be neces- 

 sary to undergo the labor of making an elaborate set of gray 

 discs ; if the experimenter is furnished with six or eight bril- 

 liantly colored discs, the reflecting power of which has been 

 carefully determined, then, with these and black and white 

 discs, he will be able to match new discs and measure their 

 luminosity in the old way. 



