ASTEOoSTOMY AND METEOEOLOGY. 



METHOD OF LOCAL CORRECTION FOR REFRACTING TELE- 

 SPOPES. BY MR. ALVAN CLARK. 



AT a meeting of the American Academy of Arts and Sciences, 

 in 1867, the Rumford Medal, awarded to Mr. Alvan Clark for his 

 improvement in telescopes, was formally presented. The pres- 

 entation was prefaced by an address by the president, Prof. Asa 

 Gray, from which the following are extracts : 



" Three-quarters of a year have passed since, by unanimous 

 vote, the Rumford premium was awarded to Mr. Alvan Clark. 

 It will be very proper, therefore, briefly to rehearse the grounds 

 upon which the Rumford committee recommended, and the 

 Academy decided, to mark by these insignia their appreciation of 

 the service which Mr. Clark has rendered to astronomical science 

 in the perfecting; of its most important instrument of research, the 

 lens of the refracting telescope. 



" The two great classes of telescopes, reflectors and refractors, 

 labor under difficulties of construction peculiar to each ; and one 

 class or the other has enjoyed the preference of astronomers, ac- 

 cording to the degree in which these difficulties have respectively 

 been overcome. 



"In the refracting telescope the obstacles are: 1, the want of 

 homogeneousness in the glass; 2, the dispersion of the substance ; 

 and, 3, imperfection in the figure of the lens. If the first of these 

 difficulties has been reduced to a minimum by the selection of the 

 best materials, then the artist has to contend with the other two. 

 He must give to the four surfaces of his compound achromatic 

 lens such shapes as will produce the maximum of distinctness and 

 the minimum of discoloration in the image of the object seen 

 through it. After all has been accomplished which can be ex- 

 pected from a faithful adherence to general formulae, the last de- 

 gree of perfection will depend on the skill of the artist. 



" Now, just where most artists have supposed that their work 

 was done, our associate has thought that his had only begun. 

 Selecting either a real star, or for greater convenience substitut- 

 ing a fictitious one, he now enters upon an examination of his 

 lens, ring by ring, ascertaining by an ingenious test which annuli 

 have too long and which too short foci. The lens is then dis- 

 mounted and retouched at the defective places, is examined 

 and retouched again and again, until his simple test assures him 

 that every portion will converge the rays to the same focus. The 

 long underground tunnel which Mr. Clark has excavated on his 



284 



