PHYSICAL PROPERTIES OF EPIPOLIC PHENOMENA. 129 



face ; and a solution of quinine in water acidulated with 

 sulphuric acid, although perfectly transparent and colour- 

 less when held between the eye and the light, exhibits, 

 if viewed in a particular direction, a lively cerulean tint. 

 These effects being supposed to be due to the conditions 

 of the surface, have been called epipolic phenomena.* 

 The careful investigation of these phenomena has 

 made us acquainted with some very interesting facts, and 

 indeed discovered to us a set of luminous rays which 

 were previously unknown. The dispersion of blue light 

 from the surf ace of some yellow glasses such as have been 

 coloured by ihe oxide of silver is of a different order 

 from that which takes place with the solution of sulphate 

 of quinine, or with the fluor spar. The first depends 

 upon a peculiar condition of the surface, while the latter 

 phenomena are due to a dispersion which takes place 

 within the solid or fluid. In addition to the sulphate 

 of quinine, and the flour spar, we obtain the same results 

 in a very marked manner by a canary yellow glass, 



* 'Afji&p<t><>>Ta. On the Epipolic Dispersion of Light, being a 

 paper entitled, On a case of Superficial Colour presented by a homo- 

 geneous liquid internally colourless. By Sir J. F. W. Herschel, 

 Bart , K.H., F.B.S., &c. An epipoliznd beam of light (meaning 

 thereby a beam which has once been transmitted through a quini- 

 ferous solution, and undergone its dispersing action) is incapable 

 of further undergoing epipolic dispersion. In proof of this the fol- 

 lowing experiment may be adduced, 



A glass jar being filled with a quiniferous solution, a piece of 

 plate glass was immersed in it vertically, so as to be entirely 

 covered, and to present one face directly to the incident light. In 

 this situation, when viewed by an eye almost perpendicularly over 

 it, so as to graze either surface very obliquely, neither the anterior 

 nor posterior face showed the slightest trace of epipolic colour. 

 Now, the light, at its egress from the immersed glass, entered the 

 liquid under precisely the same circumstances as that which, when 

 traversing the anterior surface of the glass jar, underwent epipolic 

 dispersion on first entering the liquid. It had, therefore, lost a 

 property which it originally possessed, and could not, therefore. -bo 

 considered qualitatively the same light. 'Philosophical 'Transac- 

 tions, vol. cxxxvi. 



