OF ARTS AND SCIENCES : FEBRUARY 26, 1867. 245 



for providing a fictitious star, for converting day into night, so that he 

 may examine and repolish at any hour ; it also provides a sufficient 

 space filled with homogeneous air, much better adapted for nice obser- 

 vations than the external atmosphere. All this is what is meant by 

 Mr. Clark's method of local correetion, for which the Rumford premium 

 was awarded. The late Mr. Fitz of New York also used a method of 

 local correction. But he retouched only one surface of his compound 

 lens, and was therefore obliged to leave it with such irregulai-ities as 

 must result from the local working ; as it could not be returned to the 

 tool upon which it was originally polished, without rapidly losing all 

 the advantage gained by the local treatment. 



Whenever Mr. Clark dismounts his lens and retouches it, he sepa- 

 rates the component parts, and applies his local correction to each of 

 the four surfaces. These will tend to fall back to the primitive figure, 

 but, while the elasticity of the tools and the lens together suffice to pre- 

 vent a very rapid change, under Mr. Clark's skilful and delicate hand- 

 ling he allows for and guards against this by a slight excess of local 

 working. It is, therefore, to this nice, artistic handling that our asso- 

 ciate's lenses owe, in no small degree, their pre-eminence. 



The justification of the Academy's award is found in the importance 

 and the originality of this method of local correction. 



Its importance is assured and measured by the fact that by it a lens 

 of 18 to 20 or even 24 inches' aperture is as manageable under con- 

 struction now, as 9 to 12 or 15 inch lenses lately were without it. 

 Thi'ough this invention and the consummate skill by which it is turned 

 to account, supremacy in this high art, which lingered long in London 

 under the influence of the Dollands, and then deserted its ancient home 

 for Munich, has now confessedly taken up its abode with us in Mr. 

 Clark's unpretending but most efficient establishment at Cambridge. 



The originality of the invention will hardly be questioned when it is 

 known that Foucault, as recently as 1858, tried something of the same 

 kind on the single surface of a reflector, with fine results, and then sug- 

 gested its application to lenses, in honest ignorance that this was what 

 Mr. Clark had been doing for ten years, and in the most delicate and 

 refined way. 



In the administration of Count Rumford's trust, the Academy could 

 take notice only of such work as comes within its specific terms, and 

 could adjudge his premium solely in regard to the absolute merit of 

 some particular invention. It could take no account of the difficulties 



