46 Mr Dick's Description of a New Reflecting Telescope, 



ed, it appeared to be scarcely half-polished, and its surface is 

 variegated with yellowish stains, which cannot be erazed. AVere 

 it fitted up upon the Gregorian plan, it would, I presume, be of 

 very little use, unless when a very small magnifying power was 

 applied. Yet, in its present form, it bears, with great distinct- 

 ness, a magnifying power of J 70 times, and is superior in its per- 

 formance to a 4 feet achromatic refractor, with a similar magni- 

 fying power. It exhibits very distinct and interesting views of 

 the diversities of shade, and of the mountains, vales, caverns, 

 and other inequalities of the moon^s surface. The smallest in- 

 strument I have fitted up on this plan is one whose speculum is 

 only 5i inches focal distance, and 1| inches diameter. With a 

 magnifying power of about 1 5 times, it shows land objects with 

 great distinctness and brilliancy. But I would deem it inexpe- 

 dient to fit up any instrument of this description, with specula 

 of a shorter focal distance than 10 or 12 inches. 



The following are some of the properties and advantages pe- 

 culiar to this construction of the reflecting telescope. 



1. It is extremely simple, and may be fitted up at a compara- 

 tively small expence. Instead of large and expensive brass 

 tubes, such as are used in the Gregorian and Newtonian con- 

 struction, little more is required than a short mahogany tube, 

 2 or 3 inches long, to serve as a socket for the speculum, with 

 an arm about the focal length of the mirror. The expence of 

 small specula, either plain or concave, is saved, together with 

 the numerous screws, springs, &c. for centring the two specu- 

 la, and placing the small mirror parallel to the large one. The 

 only adjustment requisite in tliis construction, is that of the eye- 

 tube to the speculum ; and, by means of the simple apparatus 

 already described, it can be effected in the course of a few mi- 

 nutes. Almost the whole expence of the instrument consists in 

 the price of the speculum and the eye-pieces. The expence of 

 fitting up the four feet speculum, alluded to above, including 

 mahogany tube and arm, brass sockets, screws, eye-tube, brass 

 joint, and a cast-iron stand, painted and varnished, did not 

 amount to L. 1, 7s. A Gregorian of the same size would have 

 required a brass tube at least 4| feet in length, which would 

 cost four or five guineas, besides the apparatus connected with 

 the small speculum, and the additional expence connected with 



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