294 



Mr. J. Brown. 



[Nov. 18, 



When the eye is looking nearly in the right direction, the spectrum 

 appears to be covered by a veil proceeding from the red end np to a 

 point dependent npon the precise direction of the light. By slightly 

 shifting the eye, the veil may be made to reach any desired part of 

 the spectrum, and then we know for what ray total reflection is just 

 commencing. By bringing the veil to touch the soda line (rendered 

 visible with the aid of the spirit lamp), precisely the same direction 

 was found as had previously been marked out with use of homo- 

 geneous light. It would be possible in this way to determine with 

 considerable accuracy the dispersive powers of opaque bodies. 



The angle of 18°, being measured in air, is not the complement of 

 the true angle of reflection. If we take 1*630 as the index of CS 3 for 

 soda light, we find for this angle 



' sin-^I?W0° 56' ; 

 V 1-630/ 



whence for the index of the glass relative to soda light, 



^=1*630 cos 10° 56'=l-600. 



The amount of reflection according to Fresnel's formula, with an 

 incidence of 13° 52' and an index 1*600, is 0*05726, a little less than 

 that actually observed. The agreement is as good as could be 

 expected, but it should be noticed that this mirror was merely 

 cleaned and not repolished with putty powder. If repolishing were 

 to produce as much effect in this case as upon the acute-angled prism 

 (I), Fresnel's formula would be ]eft considerably in arrear.* 



P.S. Nov. 9, 1886. — I am indebted to Mr. Glazebrook for a deter- 

 mination of the refractive index of the prism of crown glass II. He 

 finds fi = 1*5328. The introduction of this into Fresnel's formula 

 (0 = 13° 52') gives for the reflecting power 0*0477. 



V. " A Theory of Voltaic Action." By J. Bkown. Communi- 

 cated by Lord Rayleigh, Sec. U.S. Received October 4, 

 1886. 



[Plates 4 and 5.] 



1. From a series of experiments made more or less continuously 

 during the last five years the following conclusions have been 

 drawn : — 



That the difference of potential near two metals in contact as 

 observed either by the bi-metallic condenser (Volta's) method, 



* Some of the results here given were communicated to the British Association at 

 Birmingham, where also was read a paper by Sir John Conroy on the same 

 subject. 



