370 Br. Draper's Telescope. 



the grinding process, and an idea was entertained of saving 

 much weight by electrotyping a brass mirror with speculum 

 metal. When he commenced operations with glass, he had to 

 polish with his own hands more than 100 mirrors of various 

 sizes, from 19 inches to ■£ inch, and to experience very fre- 

 quent failures for three years before he was able to produce 

 large surfaces certainly and speedily. His labour would have 

 been much diminished, inasmuch as he would have been 

 spared the causeless condemnation of many fine mirrors, as 

 well as the working of some square ones, had he become 

 earlier aware of an important fact respecting the rigidity of 

 the material.* Generally speaking, a sheet of glass, even when 

 very thick, can hardly be set on edge without so much flexure 

 as to render it optically worthless ; but fortunately, in every 

 disc that he tried, there was one diameter on either end of 

 which it might stand without harm. On turning a disc of 15l 

 inches, with a thickness of 1 j inch, one quarter round, it could 

 hardly be realized that the surface was the same : 90° more 

 restored it to its original defining power ; and this effect was 

 found to be independent of any irregularity on the edge of the 

 disc and of the mode of support. Dr. Draper refers it with great 

 probability to the structure of the glass, resulting from its 

 having been subjected to rolling pressure. A similar irregu- 

 larity of structure is known to obtain in many large object- 

 glasses, and Dr. Draper specifies the great achromatic by 

 Cauchoix, presented by the late Duke of Northumberland to the 

 University of Cambridge, as having had its lenses turned round 

 by Mr. Airy in mounting, for this reason. Short's Gregorian 

 specula, too, were always marked on the same account. The 

 strange deformations of image produced by heat, even by the 

 warmth of the hand for a few seconds, are described and 

 represented. From such distortions the speculum would not 

 recover in ten minutes, and the error would be rendered per- 

 manent by repolishing in that condition ; and so injurious may 

 such causes, even in a lesser degree, prove during the delicate 

 process of the final correction of the spherical error, that " a 

 current of cold or warm air, a gleam of sunlight, the close 

 approach of some person, an unguarded touch, the application 

 of cold water injudiciously, will ruin the labour of days." He 

 found it a matter of not unfrequent occurrence that a speculum 

 would perform much better with rays of a certain amount of 

 obliquity, f deviating from 2° to 3° from the axis. It is obvious 



* In examining and testing last year some fine 8-inch specula of Mr. With's 

 workmanship, I had independently ascertained this peculiarity, so far as a test 

 position for each was concerned, but I stopped short of Dr. Draper's discovery o 

 a regular axis of rigidity. 



f I became acquainted with this fact many years ago, when working metals 

 for a small Newtonian reflector. 



