174 Photometer for Measuring Intensities of Lights. 



carmine and 25 of -white was on the petroleum-flame side of 

 the photometer-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 a condition to make an interesting experi- 

 ment which shows in a striking manner the effects of the 

 coloured central disks and peripheral rings on the colours of 

 the photometric 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 rings, 

 and rotate the disk in front of the mirrors. We now see the 

 photometric ring with strong contrast of colour on its sides, 

 and the concentric rings a and b are separately seen, not 

 appearing indistinguishable 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 colours on the disks and rings 

 of the photometer must not be too saturated in hue. The 

 elementary disks of these compound colour-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 colour of the photometric ring, the same hues 

 should be painted on single disks, which are subsequently 

 flattened. The thinner the colour-correcting disks and the 

 closer they fit to the photometer-disk, the greater is their 

 contrast colour-effect. The coloured 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 

 experiments so that one could get the proper hues of the 

 correcting 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 colours 

 variously saturated and repeat the experiments I have de- 

 scribed, 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, &c. when com- 

 pared 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 -dish Photometer, — 

 Ou one side of the photometer was placed " a white Welsbach 



