and the Intensities of Lights of different Colors. 21 



same hue on the periphery and a disk of 75 parts of carmine 

 and 25 of white was on the petroleum name side of the pho- 

 tometer disk, with a ring of the same hue on the periphery of 

 the photometer, that the sides of the photometric ring of the 

 photometer had the same hue ; which was white tinged with 

 just a perceptible carmine. 



We are now in condition to make an interesting experiment 

 which shows in a striking manner the effects of the colored 

 central disks and peripheral rings on the colors of the photo- 

 metric ring of the photometer. The exact balance of hue on 

 both sides of the photometer has been obtained. Reverse the 

 photometer disk with its central disks and lings and rotate the 

 disk in front of the mirrors. We now see the photometric 

 ring with strong contrast of color on its sides, and the concen- 

 tric rings a and h are separately seen, not appearing indistin- 

 guishable as in the previous experiment. By the reversal we 

 have, by the effects of the disks and rings, added to the effects 

 produced alone by the lights. 



It is to be noted that the colors on the disks and rings of the 

 photometer must not be too saturated in hue. The elementary 

 disks of these compound color disks should be made of thin 

 Bristol board and after they have been painted should be well 

 flattened before they are slit radially. After the proper hues 

 have been obtained for the disks which correct the color of the 

 photometric ring, the same hues should be painted on single 

 disks, which are subsequently flattened. The thinner the 

 color-correcting disks and the closer they fit to the photometer 

 disk the greater is their contrast color-effect. The colored 

 rings must be cut out of the painted cardboard, for if the 

 rings are painted after they are formed it is very difficult to 

 make them flat. 



To render easy the making of this photometer I made ex- 

 periments so that one could get the proper hues of the cor- 

 recting disks and rings by mixtures of definite weights of the 

 pigment powders and definite volumes of gum-water ; but it is 

 less troublesome to paint a few disks with colors variously 

 saturated and repeat the experiments I have described, and 

 thus furnish oneself with the sets of disks and rings necessary 

 for the photometric measures of the arc electric light, of the 

 Welsbach white burner, etc., when compared with the standard 

 candle or petroleum flame. 



The rotator on which the photometer-disk is revolved should 

 be made as shown in fig. 18, so that the standard, S, and the 

 pulley, P, which is driven by the wheel, W, shade as little as 

 possible the rotating disk. 



Photometric measures with the Rotating- Disk Photometer. 

 — On one side of the photometer was placed " a white Wels- 



