181 



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 A^ery little space on the lens, no points of fair trans- 

 mission may there remain; and the few rays, that pass 

 through, may be so distorted, by irregular refraction, and 

 inflection, in the glass, and in the eye-hole, that the vision 

 must be indistinct. And this was the more likely to hap- 

 pen, in Huygens's time; because, neither the fine polishing 

 powder, of colcotliar of vitriol, was then in use; (and Mr. 

 Huygens used nothing but tripoli;) nor ivas the method of 

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

 Newton. If this conjecture be right, the remedy is, to 

 use both these helps, in communicating an exquisite polish 

 to the eye-glasses; especially the smaller one, where the 

 breadth of the pencils is reduced, in the same proportion 

 as its radius, or as the increase of its magnifying power: 

 and, also, to avoid using flmt 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 

 polish of glass must ever be, to a certain degree, imper- 

 fect; so the projected improveinent of Mr. Ramsden, to 

 avoid the 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 



VOL. X. A a than 



