?) 





J) 



24-5 



5) 



13 



J) 



77 



5J 



19 



5J 



8-5 



1876.] Repulsion resulting from Radiation. 277 



sensitiveness. The whole is completely exhausted, and then enclosed in 

 a box 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 degrees 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 : — 



o 



Candle 6 feet off gives a deflection of 218 

 12 

 18 

 24 

 10 

 20 

 30 



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 



o 



Yellow glass 161 



Blue „ 102 



Green „ 101 



Red „ 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-burner 113 „ 



These experiments show how conveniently and accurately this instru- 

 ment 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 



