on the Electrical Resistance of Selenium. 217 



spectrum on a very bright cloudless day (conditions as before), 

 except that more battery-power was used (10 cells Daniell). 



The diffused daylight could not be cut off; so the trial was made 

 in the most shaded part of an ordinary room, the spectrum being 

 superimposed on the ordinary diffused daylight. 



The resistances were very carefully balanced in each case, with 

 the following results : — 



Resistance in darkness 330,000 



„ violet 279,000 



„ red 255,700 



„ „ orange 277,000 



„ green 278,000 



„ „ blue and indigo 279,000 . 



„ „ centre of red 255,000 



Resistance just on the outside edge of red, 



red side 220,000 



Resistance in dark rays clear of red 228,000 



Resistance in diffused daylight only 270,000 



Resistance taken in the dark immediately 



after exposure (resistance rising) 310,000 



The indications were very clear ; and the bar of selenium was so 

 sensitive to the action of the spectrum, that a slight movement of 

 the prism produced a corresponding movement in the spot of light 

 on the galvanometer-scale. 



It is to be noted that in this experiment the reflecting galvano- 

 meter was placed on a heavy masonry pillar insulated from the 

 floor for observatory purposes, and that the battery, bar of sele- 

 nium, and resistance-coils were in another room, being connected 

 by long and carefully insulated leads with the galvanometer. 



Experiment 4. — The diffused light was cut off as much as pos- 

 sible by screens ; and the resistances were again balanced in the 

 solar spectrum. Conditions as in the last case. 



Resistance of selenium in red 240,000 



„ just outside red 240,700 



in blue 270,000 



„ in such diffused light as came 



through screens 290,000 



Light cut off by lid of box (resistance rising) 310,000 



Experiment 5. — The selenium was also exposed to the spectrum 

 of the electric light in a darkened room. 



The effect was feeble ; but by using more battery-power in ba- 

 lancing, it was possible to measure the swing of the spot of light 

 when the selenium was suddenly exposed to the action of the light 

 of the spectrum. 



The maximum effect was obtained in, or just at the edge of the 

 red, the violet and blue rays producing scarcely any effect. 



Experiment 6.— -The selenium was exposed to the full sunlight; 

 the resistance fell enormously and instantaneously, and on balan- 



