TELESCOPE. 



values of tie micromcler'* revolution, found as above de- 

 feribed, v/en 66".o, 4y'.5, and i6".8 ; viz. exadly in the 

 fcverfe proportion of their focal lengths, as the theory re- 

 quires. Hence, when the value of the micrometer is known 

 with a telefcope of a known focal length, its value may be 

 had, when applied to any other telefcope of a determined 

 focal length, by reciprocal proportion; for as 30.15 in. : 

 6^.5 in. :: 3i".3 : 66".o ; and convcrfely, wlien the values 

 arc known, and the focal length of one of the telefcopes, the 

 focal lengths of all the others may be determined ; whicli- is 



equally the cafe with the powers, depending folely on the 

 focal lengths when the fame eye-piece is ufed witli each. 

 When the values of tlie micrometer fcrew had been deter- 

 mined, both by meafuremcnt of the fun, and by mutual com- 

 ])ar)fon of the focal lengths of the four achromatic tele- 

 Icopes, the two following tables were conftrufted to facili- 

 tate the ufe of the micrometer with any or all of the faid 

 telefcopes, which we fiibjoin as a fpecimcn by which other 

 tables may be conllrufted by the fimple arrangement of the 

 multiples of the value of unity. 



Table I. Values of entire Re\folutions of the Micrometer's Screw, with four differept Telefcopes. 



Taeli 



