Magftijying Pcti'frofTeUfccprs. ^^ 



to tnake it niare tranfparcnt. I fonietimes omit making the 

 lines 1, 3, 3, See. and content mvfclf with numbering every 

 fecond hole made by the dividers; the numbers are to ex- 

 prefs, in hundredths of an inch, the diflance of the Unes a^ b, 

 and a, c, at thofe points where they are placed. Being thus 

 provided, I adjuft the telefcope to diftinft vifion, by looking 

 at fome diftantobjeft, and then take off the eye-hole, if the 

 telefcope will admit it to come off, which refleftors always do, 

 but the night eye-pieces of achroniatics do not without taking 

 away the firft glafs alfo. Having now direiled the telefcope 

 to the open day-light, I take the divided paper in one hand, 

 and a hand magnifier to view" it with in the other, and ap- 

 ply the paper to mcafure the diameter of the bright pencil 

 of light as it emerges from the eye-glafs, and at that diftance 

 where it appears Icail in diameter, as at c, fig. 29, \\ here the 

 rays of light, having paffed the lens a, converging to c, there 

 crofs each other, and proceed diverging to d and e : this 

 place, where the paper is to be applied^ will be eafily knowa 

 by obferving, with your hand magnifier, (which (hould be 

 of an inch, or at moft an inch and half focus,) where the 

 bright fpot, as at d, fig. 28, appears diftinil and well de-- 

 fined at the edges, and the filaments of the paper di(9:in£tly 

 feen at the fame time. A place on the paper may be found, 

 where the bright fpot will juft fill up the fpace between the 

 lines a, h, and a, c. Suppofe this takes place at d, where it 

 will be extremely near the third divifion, and of courfe 

 three hundredths of an inch in diameter; and fuppofe youc 

 telefcope to be a 30-inch achromatic, and the diameter of th^ 

 objeft^lafs two inches and one tenth, or 210 fuch divifions; 

 thefe, divided by three, will give 70 for the magnifying 

 power of your inftrument. Suppofc, with a difterent eye- 

 piece, the pencil of light only occupied the fpace between 

 the lines at i, it would be one and one half of thofe divit- 

 fions: in this cafe divide 210, the hundredths of an Inch 

 fiuntaiucd in the diameter of your object ^lafs, by 1-5, and 



it 



