170 Mr. A. M. Mayer on a Photometer for Measuring 



appeared white tinted with blue. The hues were the same as 

 described in the experiment with the large disk, fig. 8, only 

 the surfaces appeared brighter by contrast with the black 

 centre 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 1 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 lamp-light side I placed a disk formed of three split 

 disks — coloured 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 lamp-light side the 

 disk gave a bluish-green. These colours 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 con- 

 trast effects of these coloured 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 

 photometer. 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 

 colours on each side of the disk. Various 

 methods were tried to render reliable the 

 comparison of the illumination of the two 

 sides. 



(1) I used the eye alone, regarding only 

 the portions of the ring on the border near 

 the mirror, as shown in fig. 14. 



(2) Two prisms reflected the images of ^\ /7 

 portions of the sides of the ring nearest \\// 

 the mirror to juxtaposition, as shown in fo.15 

 fig. 15. 



(3) A tube blackened on the inside, with a black screen on 

 the end next the disk, as shown in fig. 16, was used. The 



