Note 92 — Chapter VII

Ἁμὁρφωτα. On the Epipolic Dispersion of Light, being a paper entitled, On a case of Superficial Colour presented, by a homogeneous liquid internally colourless. By Sir J. F. W. Herschel, Bart, K.H., F.R.S., &c.—An epipolized beam of light (meaning thereby a beam which has once been transmitted through a quiniferous solution, and undergone its dispersing action) is incapable of further undergoing epipolic dispersion. In proof of this the following 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, be considered qualitatively the same light.—Philosophical Transactions, vol. cxxxvi.