MEMOIR XII.] THE MAGNETIC EFFECTS OF LIGHT. 



rated from others of the spectrum by passing through a 

 slit in a metallic screen, and half the needle was screened 

 from its action by a piece of paper. After two hours' 

 exposure to the sun it was suspended again in the ex- 

 hausted receiver, but still showed no polarity; it was 

 then exposed to the other rays successively, with the 

 same result. The needle was now slightly touched, and, 

 slowly vibrating, arranged itself in the magnetic merid- 

 ian. The first vibration was performed in a semicircular 

 arc, the number of vibrations in one hundred seconds 

 was twenty -seven. But after four hours' exposure to 

 the violet ray, as before, no evidence of any change 

 either increasing or diminishing the number of oscilla- 

 tions could be obtained. A beam of violet light passing 

 through a disk of stained glass was concentrated on one 

 end of a sewing-needle by means of a lens without pro- 

 ducing any change in the number of vibrations made in 

 one minute. This needle on some occasions would, how- 

 ever, give different results : when its first vibration was 

 performed in a semicircle, the number varied from forty- 

 one to forty-three in sixty seconds. On vibrating it in 

 vacuo, its results uniformly gave the latter number very 

 nearly. 



The position of the needle to the incident ray is not 

 of any consequence, whether it receives it obliquely, in 

 the direction of the light, or across it. If soft iron be 

 substituted for steel, the results are still negative, even 

 if the needle be arranged in the magnetic meridian, the 

 line of dip, or any other position. I therefore came to 

 the conclusion that the violet ray exerts no influence on 

 the magnetic needle, and that all the other rays are 

 equally inert. 



But as Mrs. Somerville found that a needle placed 

 under a piece of glass or blue ribbon, having half its 

 length protected by paper, became in a short time mag- 



