Prof. Powell on the Theory of the Aberration of Light, 93 



actions in different parts of the spectrum ; actions not limited 

 to the Daguerreotype, but occuriang in all kinds of cases. 

 They have been met with by every one who has examined the 

 spectrum with sensitive papers, and, in a totally different 

 series of phaenomena, M. Becquerel has himself furnished a 

 most conclusive illustration. He shows that when sulphuret 

 of lime and other phosphorescent bodies in a shining state are 

 exposed to the spectrum, the more refrangible rays increase 

 the glow, but the less extinguish it at once. What is to be 

 made of a fact like that ? 



It is proper to observe, that some of the phaenomena re- 

 corded in this communication which seem to be in opposition 

 to the principle set forth are not so in reality. All reasonings 

 founded on the decomposition of light by the prism, and the 

 action of the prismatic spectrum on changeable surfaces, are 

 liable to error. In a memoir which I shall soon publish " On 

 the Production of Light by Heat," this point will be particu- 

 larly dwelt on ; for the present it is sufficient to say, that the 

 only method free from these difficulties is to employ the inter- 

 ference spectrum formed by a ruled surface or a grate; a 

 method which was proposed eight years ago by Sir J. Her- 

 schel with a view of getting rid of the disturbing agencies 

 arising from the ideal coloration of glass, and which I first 

 carried into effect in 1844' with so much success, that the i-e- 

 sulting Daguerreotype impressions contained Fraunhofer's 

 lines, even with microscopic minuteness. With this spectrum 

 we avoid a far more serious difficulty than that of the ideal 

 coloration of glass, a difficulty arising from the magnitude of 

 the refracting faces of the prism. It is this which makes a 

 prismatic spectrum blacken paper, made sensitive with the 

 bromide of silver, from the red to the violet end ; whereas the 

 interference spectrum shows that the true action is confined to 

 the more refrangible side, and stops short of the centre of the 

 yellow space. 



University, New York, 

 Dec. 24, 1846. 



XIX. 1^ ate to a former paper on the Theory of the Aheiiration 

 of Light. By the Rev. Professor Poweli„ 



C INCE the publication of my paper in a late Number of 

 this Journal*, I have been favoured with several commu- 

 nications from the distinguished individuals on whose opinions 

 I have there commented ; and their friendly criticisms seem 

 to call for a brief supplementary remark. 



• Phil. Mag., Dec. 1846. 



