On a Simplified Dispersion-Photometer. 45 



out. On the second day we made it give out energy more 

 rapidly than the first, and on the third more rapidly than on 



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the second, this being done of course by diminishing the total 

 resistance in circuit. During the last day we were discharging 

 with a current of about 25 amperes. And this cell, like the 

 others, showed, on being insulated after having been apparently 

 totally discharged, that there was still a large charge stored up; 

 hence the numbers given above for the capacity are probably 

 under the total value. 



Deterioration. — As to deterioration, two months constant 

 charging and discharging of the two accumulators under test 

 showed no signs of deterioration. 



IV. A Simplified Dispersion-Photometer. 

 By Professors W. B. Ayrton and John Perry*. 



IT will be in the recollection of the Members that in 1879 

 we described to the Society a dispersion-photometer 

 which enabled measurements to be made of the intensity of 

 the strongest electric light in a small room and for the rays 

 coming from the electric light at any angle — two essentials 

 which appeared to us necessary in an electric-light photometer. 

 The principle of this photometer consisted in our use of a con- 

 cave lens to weaken the strength of the light, so as to make 

 the illumination of a screen comparable with the illumination 

 of a standard candle, instead of keeping the lamp a distance of 



* Communicated by the Physical Societv, having been read at the 

 Meeting on February 8-5, 1882, 



