Mr Rogers on the Construction of Achromatic Telescopes. 127 



achromatic telescope, the object of which is to render a small 

 disc of flint-glass available to perform the office of compensa- 

 tion to a larger one of crown, and thus to render possible the 

 construction of telescopes of much larger aperture than are 

 now common, without hindrance from the difficulty at present 

 experienced in procuring large discs of flint-glass. It is well 

 known that in the ordinary construction of an achromatic ob- 

 ject-glass, in which a single crown lens is compensated by a 

 single one of flint, the two lenses admit of being separated only 

 by an interval too small to afford any material advantage in 

 diminishing the diameter of the flint lens, by placing it in a 

 narrower part of the cone of rays, the actual amount of their 

 difference in point of dispersive power being such as *o render 

 the correction of the chromatic aberration impossible, when 

 their mutual distance exceeds a certain limit. 



This inconvenience Mr Rogers proposes to obviate, and ob- 

 tain the advantage in question, by employing as a correcting 

 lens not a single lens of flint, but a compound one, consisting 

 of a convex crown and concave flint, whose foci are such as to 

 cause their combination to act as a plane glass on the mean re- 

 frangible rays. Then it is evident that by reason of the greater 

 dispersive power of flint than of crown glass, this will act as a 

 concave on the violet, and as a convex on the red rays ; and 

 that the more powerfully, according as the lenses separately 

 have greater powers or curvatures. If, then, such a compound 

 lens* be interposed between the object-glass of a telescope, 

 supposed to be a single lens of plate or crown glass, and its 

 focus, it will cause no alteration in the focus for mean rays, 

 while it will lengthen the focus for violet, and shorten it for 

 red rays. Now this is precisely what is wanted to produce an 

 achromatic union of all the rays in the focus ; and as nothing 



* A compound lens of this description was proposed for the same pur- 

 pose by Dr Brewster several years ago. The construction of the lens is 

 mentioned both in the Edinburgh Transactions and the Edinburgh Ency- 

 clopaedia. The application of it to achromatic telescopes he explained to 

 an eminent individual now in a distant part of the world ; but as he never 

 published any account of this application, Mr Rogers has all the merit of an 

 original invention. Dr B. constructed several chromatic lenses, as he called 

 them, with oil of cassia, &c. and plate glass, and frequently used them for 

 correcting the colour in the eye, and also in magnifying glasses.— Er>. 



