182 



Uian a point, on the surface of the lens; and, if that point 

 should be opaque, or unpolished, or covered with dust, no 

 rays of the whole pencil could be transmitted through it: 

 which would, probably, happen to too many of the pen- 

 cils, and affect the image, which is composed of these 

 pencils; whose latitude, therefore, ought to be greater 

 than the limiting measure stated by Mr. tluygens. And, 

 'to effect (his, Avith as high a charge as the instrument 

 will bear, a part of the amplification should be thrown 

 away, on the first eye-glass, and diminished, by shorten- 

 ing its focus; that the pencils being, by it, rendered more 

 obtuse, may fall, with greater divergence, and latitude, 

 on the second eye-glass, which may thus be made shorter, 

 and on the eye. For it is not to be supposed, that the 

 image, formed in a telescope, can be viewed, by a small 

 lens, with as much advantage as an object may. The rays, 

 from each point of the latter, fall upon the whole surface 

 of the lens; and, therefore, a sufficient nvmiber of them, 

 ;to fill the pupil of the eye, must be transmitted: whereas, 

 the rays, from each point of the image, occupy only a 

 ^small speck on the lens ; in no case larger, in effect, than 

 the pupil of the eye: and must, when so contracted, be 

 the more obstructed, by any imperfections in the glass. 



As, therefore, the fineness of the powder, employed m 

 ■giving the highest lustre and polish to the specula, and 

 to the eye-glasses, is of great importance, in that process: 

 and, as I found, that the crocus, or colcothar of green 

 vitriol, (now used, as the fittest for that end,) could seldom 

 • iiui) .be 



