Observations on Achromatic and Reflecting Telescopes. 309 



those luminous bands which I have mentioned. If a method 

 is not found for remedying this defect, it will become an ob- 

 stacle to the unlimited magnifying power of telescopes, which 

 would be obtained, if we could form mirrors so as to give an 

 image as distinct as the object itself. These telescopes, in- 

 deed, would always err from want of light. 



Phenomena, analogous to those which I have described, 

 take place also in achromatic telescopes ; and the phenome- 

 non of false discs is in this case still more remarkable. The 

 image of a luminous point is now accompanied with a series 

 of concentric rings, * which are easily discovered by bringing 

 the eye-glass alternately within and without the place of dis- 

 tinct vision. The cause of those appearances appears to be 

 the same in the two telescopes, but, in the achromatic ones, 

 there is a certain arrangement which favours the production of 

 these x-ays. Experience has taught me to form double object- 

 glasses, so that I can make either one ring, or a greater num- 

 ber of rings, appear, by moving the eye-glass on each side of 

 the place of distinct vision. 



Art. XXV. — Observations on the Relative Performances of 

 Achromatic and Reflecting Telescopes. By Mr Herschel, 

 ■ Mr Smith, and Professor Amici. 



-Having already had occasion to make our readers acquaint- 

 ed with the improvements which have been made on the con- 

 tinent upon achromatic telescopes, we are glad to have an op- 

 portunity of laying before them an account of the fine achro- 

 matic telescope of M. Lerebours, together with the opinions 

 of three eminent astronomers, respecting the relative per- 

 formances of refracting and reflecting telescopes. The re- 

 marks of Mr South and Mr Herschel were addressed to Pro- 

 fessor Shumacher, and published in his Astronomische Nach- 

 richten, and those of Professor Amici appeared in the Corre- 

 spondence Astronomique of Baron Zach. 



The diameter, says Mr South, of the object-glass of the 



" Sec page 281 of this Number. 



