for shortening Reflecting Telescopes, &c. 291 



where an image is formed, and viewed with the eye-glass G. 

 By giving a small inclination to the plane speculum, the 

 image may be formed at A, above the speculum, without a 

 perforation in its centre. 



The merit of this invention does not exclusively belong to 

 the French astronomer. The same principle was employed in 

 the construction of the reflecting telescope more than six 

 years ago by Mr. Paterson, who had then the management 

 of the Observatory at Edinburgh, and instruments con- 

 structed in this manner were advertised for sale in that city.. 

 In order to try the performance of a telescope made on this 

 principle, I constructed a small one several years ago, with- 

 out perforating the concave speculum ; but the great loss of 

 light when the plane speculum intercepted the incident rays 

 that would otherwise have fallen on the central and the most 

 perfect part of the concave one, and the distortion of the 

 image when the great speculum was inclined a little and the 

 plane one placed out of the incident rays, induced me to 

 think that the convenience which arises from shortening the 

 tube could by no means compensate for the disadvantages 

 with which it was necessarily attended. This was also the 

 opinion of the late celebrated Dr. Robison, to whom Mr. 

 Paterson showed one of his new telescopes. M. Burckhardt, 

 however, seems to think that the advantages resulting from 

 shortening the tube greatly preponderate ; and that the loss 

 of light, which I have before stated, may be remedied by en- 

 larging the aperture of the concave speculum. The Board 

 of Longitude at Paris have seconded the views of M. Burck- 

 hardt, and have ordered a telescope to be constructed on the 

 principle already described. 



As this subject has been taken up by the French astro- 

 nomers, it may not be uninteresting to give an account of 

 two -.other contrivances which have occurred to me for 

 shortening the tubes of telescopes. If instead of the plane 

 Speculum C, we substitute a convex one so as to form an 

 image at O, the telescope becomes Cassegrainiam, requiring 

 a tube only about one- half the focal length of the concave 

 speculum. This construction is preferable to that which is 

 described by Surckhardt ; because it is much easier to give a 



T % correct 



