90 REFLECTING TELESCOPE. 



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 pencils, and 

 affect the image, which is composed of these pencils; whose 

 latitude, therefore, ought to be greater than the limiling 

 measure stated by Mr. Huygens. And, to effect this, with 

 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 shortening its focus; that the pencils 

 being, by it, rendered more obtuse, may fall, with greater 

 divergence, and latitude, on the second eye-glass, Avhich 

 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 num- 

 ber of them, to fill the pupil of the eye, must be tran?. 

 mitted ; whereas, the rays, from each point of the image, 

 occupy only a small specie on the lens ; in no case larger, in 

 effect, than the pupil of the eye ; and must, when so con- 

 tracted, be the more obstructed, by any imperfections in 

 the glass. 

 The common As, therefore, the fineness of the powder, employed in 

 colcothar ob- gjyjng 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 be 

 procured, so free from grittiness, as to be capable of leviga- 

 tion, to a sufficient degree of fineness ; insomuch, that I was 

 obliged to attempt to make it myself: it may be useful to 

 state, that, by the following easy method, I succeeded in 

 producing some of excellent quality. 



Considering that the vitriol, distilled in close vessels, 

 makeTllis col- ™ig^t probably contract this grittiness, in its calx, from an 

 cothar by union of some of its component parts, or principles, with 

 slowlyroasting^jjg ^atgy contained in the vitriol, (which is the metallic 

 or greJ^sut'' salt of iron,) and that this might prevent its perfect calcina- 

 phate of iron, tion ; I thought it best to perform the distiliatton in the 

 open air, and to begin with exhaling the water. Accord- 

 ingly, I commenced with slowly roasting the vitriol or cop- 

 peras, 



