Cambridge Phitosophimt Societij. 69 



iiiide of that metal, a reduction in colour ensues, and an analogous 

 result is obtained when the double sulphate of copper and potash 

 acts on the acetate of copper — facts which point to a decomposition 

 of the double salt in solution. Indeed it is evident that some double 

 salts are resolved more or less into their components b}' water, while 

 others are not so affected. 



The general tendency of my observations has led me to the opinion, 

 that water does not act upon a salt dissolved in it in a manner analo- 

 gous to that of the hydracids, but I hesitate to draw any conclusion 

 as to the rational constitution of a dissolved salt. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 

 [Continued from vol. xiii. p. 218.] 



Feb. 9, 1857. — Prof. Challis gave an account of his Observation 

 of the Occultation of Jupiter on Jan. 2, 1857. 



Feb. 23. — A paper w^as read " On the Theory of Polarized Fasci- 

 culi, commonly known as Haidinger's Brushes." By the Rev. J. 

 Power, M.A., Librarian of the University. 



In this paper the view taken of the subject is similar to that 

 which hud occurred to M. Jamin, and which will be found in Pog- 

 gendorff's Annalen, 1849, p. 145, and in the Comptes Rendus, tome 

 xxvi. p. 197. The author arrived, however, at tlie present theory 

 quite independently in the course of last summer, and before he had 

 acquainted himself with the literature of the subject. M. Jamin 

 had taken as an esscii de calciil the particular semi-visual angle 20'^, 

 which lies far beyond the limits within which the phaenomenon is 

 visible; and he has not attempted to give the general law for small 

 angles, which was the real problem to be solved. 



This is what the author has attempted in 'the present communica- 

 tion, availing himself of the experimental researches of Chossat 

 given in the Bulletin de la Soc. Philomatique, 1818, p. 94. 



The subject was rendered more complicated by the circumstance 

 that the formulae for the intensities of the refracted pencils are given 

 differently by Neumann, Airy, and the author of this paper. Instead 

 of taking any one set of formulae, the author managed to take them 

 all into consideration by previously showing that Airy's formulae 



result from Neumann's by multiplying them by ^, which is equi- 



. „ tan d 



valent to - . ; while his own result from the same by multi- 



H cos d, 



plying them by ; and 0^ being the angles of incidence and 



refraction. 



It follows from thence that, whichever set of formulre wc may 

 prefer, we shall have after refraction, in all cases, the following sim- 

 ple relation for two oppositely polarized incident pencils of equal in- 

 tensity; namely, 



intensity of pencil polarized in the plane of incidence 

 intensity of the pencil j)olarizcd at right angle.* 

 s=C08« (e— 0<). 



