Photometry of Lights of Different Colours. 851 



perfection in flicker photometers, a word may be said. Some 

 experimenters have thought they found differences in the 

 results with the flicker photometer when it was operated at 

 different speeds. As a matter of fact there is only one speed 

 for such a photometer, namely, that which just causes colour 

 flicker to disappear. Any greater speed causes a decrease 

 of sensibility. Now as the colour difference is decreased a 

 decrease in speed and an increase in sensibility are made 

 possible. If there is no difference in colour, the speed drops 

 to zero. In other words, for lights of the same colour the 

 flicker photometer approaches as its limiting case the equality 

 photometer. In the instrument now described this is proved 

 to be literally so. By proper choice of voltages on the test 

 and comparison lamp, and by slightly tinging the light from 

 the latter by letting it fall upon a flashed opal glass over 

 which is laid four thicknesses of lantern-slide cover-glass, 

 the total spectral light, obtained from the wide slit, and the 

 comparison standard light, are identical in appearance. The 

 speed for greatest sensibility is as it should be, zero. 



Another detail of some importance is that in measuring 

 the total light through the prism a neutral tint screen * is 

 placed over the slit, of such transmission that it reduces this 

 light to approximately the same brightness as the comparison 

 standard, the exact setting being made by means of the 

 variable neutral tint screen before mentioned. The object 

 of this is to have all the measurements carried on at the 

 same brightness, although since this final measurement is 

 of lights of the same colour it is not perhaps absolutely 

 necessary. 



It may fairly be claimed for this arrangement of apparatus 

 that physically the whole is exactly the sum of the parts. 

 There is no measurement of areas ; all scattered light is 

 present exactly in the same manner in the fractional and 

 total light measurements ; there are no instrumental changes 

 for which correction need be made, such as the removal of 

 the prism in Tufts' experiment. 



Three sets of measurements were made. For each an 

 illumination of 300 I.U. held ; the field was 2° diameter, 

 and surrounded by a large bright area (25° diameter). Two 

 sets were made by the writer, whose luminosity curve lies 

 nearly at the blue extreme of the observers previously 

 examined, one set by Dr. P. W. Cobb whose luminosity 

 curve lies at the red extreme. 



* "A Form of Neutral Tint Absorbing- Screen," H. E. Ives & 

 M. Luckiesh, Pbys. Rev. May 1911. 



