528 



Mr, Shelf ord Bidwell 



[March II 



cell has attained this condition, it be removed from the hot metal 

 plate, I have little doubt that on testing it with a galvanometer, it 

 will be found to conduct electricity and to be sensitive to light. (Exp.) 

 According to Professor Graham Bell, nothing more is necessary for 

 obtaining the greatest degree of sensitiveness. The old-fashioned 

 process of long heating and slow cooling may, he says, be altogether 

 dispensed with. In this matter my experience differs entirely from 

 his, for I find that cells which have been kept for some hours at a 

 temperature just below the point of fusion, and then allowed to cool 

 very gradually, are vastly more sensitive to light than those which 

 have not been thus annealed. 



The following table shows the resistances in the dark, and under 

 different degrees of illumination, of a few cells taken from my stock. 

 The resistance of No. 6, when exposed to a lime-light at 10 inches, is 

 less than one-fiftieth of its resistance in the dark. 



Resistances in Ohms. 



It is an interesting question, which of the coloured components of 

 white light has the greatest power in effecting these changes in the 

 resistance of selenium ; or, again, whether the effect is produced by 

 light at all, or is due simply to heat. Captain Sale came to the con- 

 clusion, on moving a piece of selenium through the solar spectrum, 

 that the maximum effect was produced at or just outside the extreme 

 end of the red, at a point nearly coinciding with the maximum of the 

 heat rays. Professor Adams performed the same experiment with 

 the spectrum both of the sun and of the electric light, and found that 

 the action on the selenium was greatest " in the greenish-yellow and 

 in the red portions of the spectrum." The greenish-yellow is the 

 point of maximum illumination, which is a remarkable fact ; but his 

 words seem to imply that there was a second maximum in the red. 

 The violet and the ultra-violet rays, he says, produced very little, 

 if any, effect. In consequence of the discrepancies in these results, I 

 determined to repeat the experiment for myself. The source of light 



