REFLECTING TELESCOPE. 0^ 



to observe, that the latitude of the pencil, as it eniers the 

 eye, is the same as that with which it falls on the last eye- 

 glass ; and, that the effect of it, which Mr. Huygens attri- 

 butes to the eye, may, therefore, as naturally, be attributed 

 to the eye-glass, as to the eye*; especially when an anato- 

 mical dissection of it will demonstrate, that the perfect 

 transparency of its humours, and exquisite polish of its cor- 

 nea, or outer coat, (not to talk of its achromatic property,) 

 far exceed, in this optic instrument of the Deity, any thing 

 that can be manufactured by man. In fact, the polish given 

 to the eye-glass, and the transparency of the glass itself, is 

 always imperfect; and many points in its surface, which 

 ought to serve for the regular transmission of light, will be 

 obstructed by its roughness and opacity ; so that, if the 

 pencils occupy but a very litde space on the lens, no points 

 of fair transmission may there remain ; and the few rays, 

 that pass through, may be so distorted, by irregular refrad 

 tion, and inflectiouj in the glass, and in the eye-hole, that the 

 vision must be indistinct. And this was the more likely to 

 happen, in Iluygens's time ; because, neither the fine polish- 

 ing powder, of colcothar of vitriol, was then in use; (and 

 Mr. Iluygens used nothing but tripoli;) nor was the method 

 of polishing, by the help of pitch, divulged by Sir Isaac 

 Newton. If this conjecture be right, the remedy is, to use ^^oVoSt" 

 both these helps, in communicating an exquisite polish to these bhould be 

 the eye-glasses; especially the smaller one, where the '1"?'"°''^'^ ^*. ? 

 breadth of the pencils is reduced, in the same proportion as anTpSish." 

 Its radius, or as the increase of its magnifying power; andj 

 also, to avoid using flint glass for this purpose; 'as it is 

 found to be the least transparent of any, as well as most 

 dispersive. 



I may here also observe, that, as the transparency and Hence Rams- 

 polish of glass must ever be, to a 'certain degree, imperfect; den's eye-pkce 

 so the projected improvement of Mr. Ramsden, to avoid the ^^ ^^J^"^^^"^ ^^• 

 dispersion of the rays, by throwing the image just before 

 the first eye-glass, is unlikely to answer: because, in this 

 case, each of the pencils would occupy little more than 



* When a white ground is viewed through a telescope which gives 

 a very slender pencil of rays, the want of transparency in the parts of 

 the eye is seen by spots and other figures, which move along with that 

 •rgaa while the telescope is Icept motionless. — N. 



a point, 



