Examination of Optical Glafs by the Magnijiei'. l8r 



parallel to the tube, and be conduced each through a flit in a brafs cap which (hall lead 

 them acrofs the aperture in a radial direftion. It is probable alfo that the artift who {hall 

 carry thefe hints into eflFeft, may alfo think of feveral other methods. 



2. ImperfeBlons of Optical Glafs. 



THE only fure method of afcertaining whether glafs be fit for optical purpofes, is to 

 give it the true figure of a lens, and examine this by the teft; of a high magnifying power. 

 It is defirable however to make fome feleftion by trial previous tor this labour. A lens may 

 be fuppofed to confift for the moft part of a fubftance of uniform refraftive denfiry, but 

 more or lefs mixed with glafs of another kind, which exhibits the appearance of fpots or 

 veins which interrupt the regular courfe of the light. Where this interruption is produced 

 by confiderable malTes, the iniperfeftions may be feen by the naked eye ; but where the 

 mafl'-s are fmaller, the glafs may appear very clear, and yet be lefs fit for optical purpofes 

 than fuch as may contain feveral vifible imperfeilions. From thefe confiderations it ap- 

 peared probable to me, that glafs might be advantageoufly examined by the magnifier ap- 

 plied near its furface. 



An exceedingly clear, brilliant, yellow-tinged, plano-convex lens, of about four inches 

 focus, was examined by looking at a candle through it, and a lens of half an inch focus 

 next the eye. When the candle was about two feet diftant, I obferved a prodigious num- 

 ber of fmall ere£l: images of candles of various fizes, which became fpots at a greater dif. 

 tance of the large lens from the eye. Removing to the diftancc of feven or eight feet 

 from the candle, the fmall images became luminous fpots, fome of them furroilnded by 

 circles of alternate light and darknefs. At this diflance, the fpots, by removing the large 

 lens further from the eye, were rendered much more perceptible. 



Several other lenfes were viewed, which exhibited a few images ; but in a white com- 

 mon fpe£lacle glafs of twelve inches focus, the images appeared fo numerous, that the lens- 

 refembled the waved window-glafs, and, at a diftance, appeared prodigioully fpotted. This 

 lens had nothing of an opake appearance, but the light pafl'ed through it too irregularly 

 to give a diftindl pifture in the camera obfcura. 



A good greenilh objecl-glafs of fix feet focus, which has a large vein or two acrofs it, 

 produced no more elTcft in thefe circumftances than if it had not been interpofed between 

 the eye and the candle. 



From the circumftances of thefe experiments, we may infer that the images are produced 

 by fpherical particles of lefs denfity than the reft of the glafs. We may fuppofe them to 

 have been filex, the fpecific gravity of which is about 2.7, which is nearly the fiime as that 

 of green or crown glafs, but is one-fifth lefs than that of flint-glafs taken at 3.2. 



When the full moon was looked at in this manner, through the firft-mentioned lens, the 

 Images were confidernbly diftinft, and all nearly of the fame fize. Their magnitude was 

 about one two-hundredth part of an inch, or one-fifth part of the fize of a bubble in the 

 glafs, which, by eftimatc, might have a diameter of one-fortieth of an inch. 



3. Purif cation cf Mercury. 

 MERCURY which is confiderably adulterated may be moll conveniently and effec- 

 tually purified by diftillation. But many pcrfons, fuch m mathematical Inftrument-makers, 



water- 



