244 PROCEEDINGS OF THE AMERICAN ACADEMY 



the completion of the Medals awarded to Mr. Alvan Clark, 

 and placed them in the hands of the President. In present- 

 ing them to Mr. Clark, the President addressed the Academy 

 as follows : — 



Gentlemen of the Academy : — 



Three quarters of a year have passed since, by your unanimous vote, 

 the Rumford premium was awarded to Mr. Alvan Clark. It will be 

 very proper, thei'efore, now that the gold medal and its silver duplicate 

 are before us, briefly to rehearse the grounds upon which the Rumford 

 Committee recommended, and you decided, to mark by these insignia 

 your appreciation of the service which Mr. Clark has rendered to as- 

 tronomical 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, according to the de- 

 gree in which these difl&culties have respectively been overcome. 



In the refracting telescope the obstacles are, — 1. the want of homo- 

 geneousness 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 a 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 pro- 

 duce the maximum of distinctness and the minimum of discoloration in 

 the image of the object seen through it. After all has been accom- 

 plished which can be expected from a faithful adherence to general 

 formulae, the last degree 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 substituting 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 dismounted 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 

 grounds, communicating with his workshop, is an original contrivance 



