148 Royal Society:— Mr. W. Crookes on 



lined with black velvet, with apertures for the rays of light to pass 

 in and out. A ray of light reflected from the mirror on to a 

 graduated scale, shows the movements of the pith bar. The de- 

 grees of deflection produced by the light of a candle at distances 

 from 6 feet to 35 feet are given. 



The experimental observations and the numbers which are 

 required by the theoretical diminution of light with the square 

 of the distance are sufficiently close, as the following figures 

 show : — 



Candle feet off gives a deflection of 218 

 „ 12 „ „ 54 



„ 18 „ „ 24-5 



» 24 ,, „ 13 



„ 10 „ „ 77 



„ 20 „ „ 19 



„ 30 „ „ 8-5 



The effect of two candles side by side is practically double, 

 and of three candles three times that of one candle. 



The action of various solid and liquid screens is next given. 

 A candle 3 feet off, giving a deflection of 180°, has its action 

 reduced to the following amounts by 



Yellow glass 161 



Blue „ 102 



Green „ 101 



Bed „ 128 



Water 47 



Alum 27 



A candle on each side of the apparatus, and equidistant from it, 

 keeps the index ray of light at zero ; by shading off either one or 

 the other the light flies off to either extremity of the scale. This 

 gives a ready means of balancing two sources of light one against 

 the other. Thus, retaining the standard candle 48 inches off on 

 the left of the bar, the index was brought to zero by placing on 

 the right 



2 candles 67 inches off, 



1 candle behind solution of sulphate of copper 6 „ 



„ „ alum plate 14 „ 



A small gas-buruer 113 „ 



These experiments show how conveniently and accurately this 

 instrument can be used as a photometer. By balancing a standard 

 candle on one side against any source of light on the other, the 

 value of the latter in terms of a candle is readily shown ; thus in 

 the last experiment the standard candle 48 inches off was balanced 

 by a gas-flame 113 inches off. The lights were therefore in the 

 proportion of 48 2 to 113 2 , or as 1 to 5^. The gas-burner was 

 therefore equal to 5| candles. 



By interposing screens of water or plates of alum, and so prac- 

 tically cuttiug off all the dark heat, the actual luminosity is measured. 

 In addition to this, by interposing coloured glasses or solutions, any 



