48 Mr Dick's Description of a New- Reflecting Telescope, 



fall upon a reflecting surface, is lost by a second reflection. The 

 image of the object may also be presumed to be more correct, as 

 it is not liable to any distortion by being reflected from another 

 speculum. ^ 



5. There is less tremor in these telescopes than in Gregorian 

 reflectors. One cause, among others, I presume, of the tremors 

 which afffect Gregorians, is the formation of a second image at a 

 great distance from the first, besides that which arises from the 

 elastic tremor of the small speculum, when carried by an arm 

 supported only at one end. But, as the image formed by the 

 speculum, in the aerial telescope, is viewed directly, without be- 

 ing exposed to any subsequent reflection, it is not so liable to 

 the tremors which are so frequently experienced in other reflect- 

 ing telescopes. Notwithstanding the length of the arm of the 

 four feet telescope above mentioned, a celestial object appears re- 

 markably steady, when passing across the field of view, especial- 

 ly when it is at a moderate degree of altitude ; and it is easily 

 kept in the field by a gentle motion applied to the arm of the 

 instrument. 



The specula used in all the instruments to which I have alluded 

 above, are far from being good, being of a yellowish colour, and 

 some of them scarcely half polished. They have likewise large 

 holes in their centre, as they were originally intended for Grego- 

 rian telescopes. Were they fitted up in the Gregorian form, 

 they would be of little use, unless with small degrees of magni- 

 fying power. Yet, with all these imperfections, they exhibit the 

 object with more brightness and accuracy than the generality of 

 reflectors ; and therefore I have no doubt that, were instruments 

 of this construction fitted up with specula of the best figure and 

 polish, they would equal, if not surpass in brilliancy and distinct- 

 ness, the general run of achromatic telescopes. 



In prosecuting my experiments in relation to these instru- 

 ments, I wished to ascertain what effect might be produced by 

 using a part of a speculum instead of the whole. For this pur- 

 pose, I cut a speculum, three feet in focal length, through the 

 center, so as to divide it into two equal parts, and fitted up each 

 part as a distinct telescope ; so that I obtained two telescopes 

 from one speculum. In this case, I found that each half of the 

 speculum performed as well as the whole speculum had done 



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