158 Royal Society. 



On making the flame luminous, the needle was suddenly de- 

 flected off the scale with great rapidity. 



With the 10 shunt to the galvanometer there was no deflection 

 on exposure to the ordinary Bunsen flame ; but with the luminous 

 flame there was a sudden deflection, which increased to 250 divi- 

 sions of the scale in a few seconds. 



This corresponded to a change of resistance in E of about 1250 

 ohms. 



This experiment was repeated in a slightly different way. The 

 selenium was balanced ; and before exposure to the luminous Bun- 

 sen flame, E was diminished by 1000 ohms. On making contact 

 and exposing at the same instant, there was a slight deflection, 

 showing that the sudden effect was equivalent to rather less than 

 1000 ohms ; but in a very few seconds the needle was at rest at 

 zero ; and to keep the needle at zero the resistance was further 

 diminished by 300 ohms. 



The resistance had been diminished by one fourth of its whole 

 amount in less than one minute in consequence of the exposure. 



Exposure to an ordinary wax taper diminished the resistance 

 of the selenium by 300,000 ohms, or about one eighth part of its 

 whole resistance. 



The illuminating powers of these sources of light were com- 

 pared by means of the Bunsen photometer. 



The light of the ordinary Bunsen flame could scarcely be 

 measured, but was somewhere about -jJtj- part of a candle, and of 

 the luminous Bunsen flame about 10 candles, whilst the light from 

 the taper was at its best rather more than one fourth of a candle. 



The heating effects of these three sources were compared by 

 means of the thermo-electric pile and delicate astatic galvano 

 meter. 



At a distance of one foot from the face of the pile the de 

 flection produced by the ordinary Bunsen flame, was 46|°and by 

 the luminous Bunsen flame was 52°, whilst the taper produced no 

 effect which could be measured. 



These experiments clearly show that very little effect is pro- 

 duced by the radiation of obscure heat, but that the effect is due al- 

 most entirely, if not entirely, to light. 



As the effects produced were measured in deflections of the 

 needle, some series of experiments were made to determine th 

 value in resistances equivalent to the divisions of the scale, fro 

 which it appeared that with the 10 shunt to the galvanometer an 

 with 30 cells, 20 divisions of the scale were equivalent to 10 

 ohms resistance; and without the shunt, 100 divisions of th 

 scale were equivalent to from 100 to 110 ohms. 



Experiments were also made to determine whether moonligh 

 would produce any change in the electrical resistance of selenium 

 The experiment was made at the half-moon, when the moon wa: 

 high up, so that the light fell obliquely on the window and di 

 not shine directly on the selenium. 



On throwing the moonlight on the selenium by means of a plan 



