M. F. ZoUner on the Spectroscopic Reversion-Telescope. 4i\7 

 The equation is as follows: — 



^ -0000002 Zm 



^~ 27 — . — \2~' 



ar\r-\-sYc 

 in which 



c = the specific conductivity of the wire, that of pure copper 



at 0° C. being unity, 

 / = the average length of a convolution in inches, 

 m = area in square inches of a semisection of the coil (that is, 



the sectional area of the space to be filled with wire), 

 s = thickness, in inches, of the insulating covering, 

 G = the resistance of the coil, in ohms, 

 r = the radius of the wire, in inches, 



« = a constant depending on the method of coiling the wire, 

 but generally =4. 

 Let 



7 = -0000001 ; 



d = the diameter required, in inches. 

 Then 



G=.;.^^> (6) 



2r'^{r-\-sfc 





>'i 



V V 2Gc 4 2 ^'^ 



As s is generally very small, we may write this equation 



'~V^Wc~%^ (^) 



and .*. 



'='</W-^- (^) 



LI. On the Spectroscopic Reversion-Telescope. 



By F. ZOLLNER*. 



[With a Plate.] 



A YEAR since, I had the honour to communicate to the Royal 

 Society of Saxony the successful application of my reversion- 

 spectroscope to the observation of the rotation of the sunf- In 

 endeavouring to introduce the extraordinarily delicate principle 



* Translated from a separate copy, communicated by the Author, from 

 the Berichte der K. Sachs. Ges. der Wiss. July 1, 1872. 



t See Phil. Mag. S. 4. vol. xhii. p. 47. 

 Phil. Mag. S. 4. Vol. 44. No. 295. Bee. 1872. 2 E 



