192 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII. 



the needles were not sufficiently exact. They found the 

 oscillation of the needles to be wholly unaffected by ex- 

 posure to the prismatic colors (JZrewster^s Journal, Vol. 

 II., p. 225, N. S.). This must still be regarded, there- 

 fore, as one of the disputed points in science" (Turner's 

 Chemistry). 



It has been supposed that these contradictory results 

 arose entirely from local circumstances. A hazy atmos- 

 phere, such as is met with in the northern and middle 

 countries of Europe, might perhaps influence in some 

 manner this peculiar property of light when the clearer 

 sky of Italy permitted the experiment to succeed. Some, 

 indeed, have thought that the observers who were said 

 to have verified the alleged results were deceived in not 

 having previously ascertained the magnetic state of the 

 needles they used. 



During the past summer (1834) I have attempted to 

 satisfy myself whether the more refrangible rays really 

 exert any magnetic influence; and happening to reside 

 in the south of Virginia, on the same parallel of latitude 

 as Tunis and the more northerly African kingdoms, I 

 thought the situation too favorable to suffer such an op- 

 portunity to pass without endeavoring to gain some in- 

 formation on this contested point. 



In 1824 Mr. Christie found that a needle six inches 

 long, contained in a brass compass -box with a glass 

 cover, suspended by a hair and made to vibrate alter- 

 nately shaded and exposed to the sun, came to rest much 

 sooner in the latter than in the former case. That this 

 was not occasioned by an increase of temperature was 

 proved by the needle vibrating more rapidly when its 

 temperature was raised by other means. 



On repeating this experiment, I very quickly found 

 that it depended in a great measure on the mode of 

 suspension of the needle, and its position with respect to 



