Dr, Pearson's Introduction to Practical Astronomy. 135 



to be furnished with a stand that will insure steadiness to the tele- 

 scope while in use ; for where there is any tremulous motion in the 

 telescope, the distinctness of vision is completely destroyed. In this 

 Section the best methods of supporting the telescope are detailed by 

 engravings. In §. X. is given an explanation of the construction of 

 a more useful stand, viz. the equatorial stand ; by means of which 

 a star can be followed by the telescope without altering its elevation, 

 or more strictly its inclination to the polar axis. 



We come now in §. XI. to the description of a useful and elegant 

 little instrument, the Dynameter. The manner of applying this is 

 very simple, and the following short account of its construction will 

 render it quite intelligible. The object-glass of a telescope is itself 

 considered as the object placed before the eye-lens at a distance 

 = F+yfroni its centre (F and^/ being the solar focal lengths of the 

 object- and eye glass respectively), and consequently an image of the 

 object-glass will be formed by the eye-lens at some distance = (5 from 

 its centre. Let D = diameter of object-glass, and d = diameter of 



F 



its image. Then the power of the telescope (Pj = ., 



F F J- f 

 But by a principle in Optics - = — £*£ 



J d 



and by similar triangles T = .v P = , 



da a- 



Now we can measure D exactly : if then we can also measure d, we 

 have a much more certain and easy method of obtaining the value 



F 



of P than is furnished by the equation P = —: The Dynameter 



enables us to measure it exactly. 



In §. XII. we find a description of the large refracting telescope 

 made by the celebrated Fraunhofer of Munich, and preserved in 

 the Imperial Observatory at Dorpat. 



Our author having described the refracting telescope, now comes, 

 in §. XIII., to explain the construction of the four kinds of Re- 

 flecting Telescopes. 



In §. XIV. we find an account of various kinds of stands for re- 

 flecting telescopes. That which appears most deserving of notice, 

 as best calculated for giving stability to a heavy Gregorian or Cas- 

 sigranian telescope, was constructed by Mr. Tulley. The contri- 

 vances in this stand for producing quick and slow motions are in- 

 deed truly beautiful. After describing some other kinds of stands, 

 the author concludes this section, by giving some rules to be ob- 

 served in the adjustment of the different parts of a Newtonian re- 

 flecting telescope. 



In §. XV. we have an account of an instrument, contrived, and 

 with indefatigable perseverance finished, by one of the most extra- 

 ordinary men that ever lived ; — this is the great 40-feet reflector of 

 Sir William Herschel. Besides the description of this instrument, 

 is given one of Sir William Herschel's curious calculations relating 

 to the time that it would take to (what he terms) sweep the visible 



