the Intensities of Lights of different Colours* 



169 



complementary colours gradually blend, and when the velocity 

 of rotation has banished all flickering light from the disk it 

 appears nearly white. The side facing the daylight has a 

 slight yellowish tint ; the side facing the lamp appears whiter, 

 but is tinted with a feeble bluish hue. 



To study more minutely these phenomena I made a disk 

 which could be readily taken apart and mounted with different 

 translucent papers and have attached to it disks and rings of 

 various colours. I shall call this disk the photometer-dish It 

 is made as follows : — 



Two disks, 13 centim. in diameter, and having eight sectors 

 cut out, as shown in fig. 13, 

 were made of thin Bristol board. 

 A circle of 2 centim. in diame- 

 ter was left in the centre of 

 the disk, from which the card- 

 board sectors radiate. The 

 border of the disk is a ring 

 of ^ centim. wide, which was 

 painted black. Clamps, made 

 of thin hammered brass, held 

 these disks together. 



Between these disks was 

 placed a circle of the same 

 white translucent paper as was 

 used in the construction of the 

 large contrast-colour screen, 



fig. 1, and the disks were clamped together with the open sectors 

 of the two disks coinciding in position. A black disk of 8'2 

 centim. in diameter was placed on each side of the photometer 

 disk, thus leaving between it and the black peripheral ring 

 an annular space of 1*9 centim. wide, formed of alternate 

 spaces of cardboard and of translucent paper. The disk was 

 mounted on a rotator and placed opposite two silvered mirrors 

 inclined at an angle of 150° ; an arrange- 

 ment similar to that of Letheby for ob- 

 serving the disk of Bunsen's photometer. 

 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 simultaneously, 

 or, rather, in rapid succession. On ro- 

 tating the disk while illuminated by day- 

 light on one side and by lamp-light on the 

 other, the side illuminated by daylight 

 appeared white tinted with yellow ; the side facing the lamp 



Phil. Mag. S. 5. Vol. 36. No. 219. Aug. 1893. N 



Fis?.13, 



Fig.14. 



