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VIII. Some Ohservations on the Amount of Liglit Rejieeted and Transmitted hy 
Certain Kinds of Glass. 
By Sir John Conroy, Bart., M.A., Bedford, Lecturer of Balliol College, and MiUard 
Lecturer of Trinity College, Oxford. 
Communicated hy A. G. Vernon Harcourt, LjL.D., F.R.S. 
Received November 8,—Read December 6, 1888. 
[Plate 8.] 
Introduction. 
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. PoBiNSON, 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 formulse, and then 
calculated the values of the extinction coefficients from the expression 
n — (log — log I)/(^ X modulus), where I is the intensity of the emergent light, 
t the thickness of the glass, and p® the coefficient giving the amount which escapes 
reflection at the two surfaces. 
Mr. Rood published in the ‘American Journal of Science’ for 1870 (vol. 50, p. 1) 
an account of some observations he had made, with a modification of Bunsen’s 
photometer, of the loss which light suffers in passing through crown glass. He states 
that the formula) for reflection were originally given by Young, and obtained subse¬ 
quently by Poisson and Fresnel, but believes that no accurate experiments have 
ever been made to test their truth. He used two plates of glass, '15 mm. and 
1'677 mm. thick, and assumed that with such thin plates the loss of light was 
practically entirely due to reflection. Fie found that with one plate, of which the 
index was 1'5236, and which therefore should have transmitted 91'736 per cent, of 
30.5.89 
