VIII. Some observations on the amount of light reflected and transmitted by certain kinds of glass

Although for both theoretical and practical purposes it is important to know the amount of light reflected from the surface of glass, and the loss which light suffers in passing through glass, but few accurate experiments appear to have been made on this subject. Dr. Robinson, in the report on the Melbourne telescope (‘Phil. Trans.,’ 1869, p. 127), gives an account of experiments made by Lord Rosse, Mr. Grubb, and himself to determine the amount of light transmitted by telescopic object glasses, and through various kinds of flint and crown glass; he states that Lord Rosse’s and Mr. Grubb’s experiments were made with a Bunsen’s photometer, and his own with a Zollner’s photometer. Dr. Robinson assumed the truth of Fresnel’s formulæ, and then calculated the values of the extinction coefficients ϵ-nt from the expressionn = (logρ 2 — log I )/(t X modulus), where I is the intensity of the emergent light,t the thickness of the glass, andρ 2 the coefficient giving the amount which escapes reflection at the two surfaces.