an 



d the Intensities of Lights of different Color 



19 



three thicknesses of paper, with 

 the same central colored disks, 

 used in the previous experiment, 

 when one side of the disk was 

 illuminated by lamplight, the 

 other by daylight. But on 

 increasing the saturation of the 

 hues of the central disks and 

 adding peripheral rings of the 

 same hues, I succeeded in mak- 

 ing the hues the same on both 

 sides of the photometer ring. If 

 equality of hoe can be obtained 

 when the photometer is illumi- 

 nated, on one side by lamplight 

 and on the other by daylight, 

 then the contrast-colors may be brought to the same hue 

 when the photometer is illuminated on one side by a candle or 

 petroleum flame and the other side by the electric arc light or 

 by the whitest Welsbach incandescent lamp. 



With a change of distance .of the petroleum flame from the 

 photometer the differences in the illumination of the portions 

 a and &, fig. 17, of the ring were changed. When the two 

 sides of the ring were equally illuminated the rings a and b 

 appeared indistinguishable, fusing into one ring of uniform 

 tint and illumination, of the breadth of a + b. 



With the best Bunsen photometer disk* I was unable to 

 decide where it received equal illumination on its sides, so diffi- 

 cult was it to judge of equal brightness of the blue and orange 

 on one side when compared with the orange and blue in the 

 same respective positions on the other side of the disk. 



With a petroleum flame of 40 candle light-giving power on 

 one side of the rotating photometer and a candle on the other, 

 the delicacy of the indications of the rotating photometer 

 equalled, and even slightly excelled, those of the very best 

 Bunsen photometer disk. 



If we bring in succession the translucent sectors of the rota- 

 ting photometer disk between two lights we observe that, on 

 certain sectors, a and b appear as one surface of the same uni- 

 form tint and illumination ; on other sectors sometimes a, 

 sometimes b, is the brighter, flotation of the disk entirely 

 destroys such slight differences and the disk then acts as if 

 made of absolutely homogeneous material, placed in the same 

 conditions of contact of the layers of paper, in each translu- 

 cent sector. 



* The best Bunsen photometer disks I have used are those sold by the Ameri- 

 can Meter Company of X. Y. They are made in England. 



