and the Intensities of Lights of different Colors. 17 



was mounted on a rotator and placed opposite two silvered 

 mirrors inclined at an angle of 150° ; an arrangement similar 

 to that of Letlieby for observing the disk of Bunsen's pho- 

 tometer. The plane of the disk of the rotator bisected the 

 angle formed by the mirrors, as shown in 

 fig. 14, so that the surfaces of both sides of 

 the disk could be seen simultaneous] y, or, 

 rather, in rapid succession. On rotating the 

 disk while illuminated by daylight on one 

 side and by lamplight on the other, the side 

 illuminated by daylight appeared white 

 tinted with yellow ; the side facing the lamp 

 appeared white tinted with blue. The hues 

 were the same as described in the experiment ng.14. 



with the large disk, fig. 8, only the surfaces 

 appeared brighter by contrast with the black center and 

 border. 



The feeble character of the hues on the photometer disk 

 led me to hope that I could bring these to the same hue on 

 both sides of the disk by the effects of contrast. To accom- 

 plish this I placed on the daylight side of the photometer- 

 disk a compound disk formed of three split disks, one of red 

 lead, one of chrome yellow, the other of white Bristol board. 

 On the lamplight side I placed a disk formed of three split 

 disks, colored ultramarine, emerald green, and one of white 

 Bristol board. On rotation, the compound disk on the day- 

 light side gave an orange yellow ; on the lamplight side the 

 disk gave a bluish green. These colors corresponded in hue 

 to those seen on the respective sides of the ring of the pho- 

 tometer disk when it is rotated, only the hues were much more 

 saturated. After a few trials I brought, by the contrast 

 effects of these colored disks, the tints on the two sides of the 

 ring of the photometer to the same hue ; the translucent ring 

 appearing white with a very slight orange yellow tint on 

 both of its sides. The greenish blue had by contrast effect 

 obliterated the blue tint of the ring and even replaced it by a 

 very feeble orange yellow ; while on the other side of the ring 

 the orange yellow disk had diminished the orange yellow tint 

 of the ring to the same feeble orange yellow as seen on the 

 other side of the ring. 



Experiments were now made to test this apparatus as a pho- 

 tometer. Equality of illumination cannot be determined with 

 any precision if the two surfaces compared differ even slightly 

 in hue. Here we had the same hue to deal with on both 

 sides of the ring ; which condition is different from the 

 Bunsen photometer on which we have two colors on each side 



Am. Jour. Sci.— Third Series, Vol. XLYI. Xo. 271.— July, 1893. 

 2 



