380 Mr. Rogers on Achromatic Telescopes. 



by the methods proposed, the construction of the correcting 

 lens would certainly demand that great care which Signor 

 Santini considers necessary; for it would require, as in the 

 ordinary construction of an achromatic, a very nice adaptation 

 of the foci and curvatures of the lenses to the refractive and 

 dispersive powers of the glass. But the changes of position, 

 by changing the apertures, and, consequently, the aberrations 

 of the lenses of the correcting glass, give us so great a command 

 over these aberrations, that this construction neither requires a 

 very accurate knowledge of the refraction and dispersion of the 

 glass employed, nor a great nicety in the foci and curvatures of 

 the lenses ; and, therefore, it is much easier in practice than 

 the common construction. 



This T have verified by actual experiment ; having had a 

 telescope made upon this principle, calculating from the ave- 

 rage values of refraction, assuming a dispersive ratio somewhat 

 lower than is met with, and making the focus of the concave 

 lens superabundantly short. Having, by these means, given 

 to the correcting lens a surplus of concave spherical aberration, 

 and a surplus of dispersive power, I was enabled, by varying 

 the positions of the lenses, to reduce these surplus aberrations, 

 so as to render the performance of the telescope not inferior to 

 that of an achromatic of the ordinary construction. The object 

 lens is of crown glass, 61 ^ inches focus, and 5 inches aperture; 

 the convex of the correcting lens is also of crown, the radius of 

 each surface 9 inches, and the aperture 3 inches ; the concave 

 is of flint glass, the radius of the first surface 9 inches, that of 

 the second surface 10.4 inches; the distance of the correcting 

 lens from the object-glass is 25.1 inches ; and the separation 

 of the lenses 0.16 inch. The correcting lens acts as a concave, 

 extending the focus to 64.7 inches. The two convex lenses 

 were originally of plate glass ; but they were so veiny and 

 imperfect in other respects, that I had them replaced by two 

 others of crown glass, made by Mr. Dollond, in whose hands the 

 telescope was a short time ago. 



The danger of deranging the centering of the lenses, by the 

 minute separation required, does not appear insurmountable 

 by very simple mechanism ; two tubes, sliding steadily upon 

 one another by the action of a screw, seem sufficient for the 



