TEANSACTIONS OF SECTION A. 527 



5. Second Report of the Committee on Standards of Light. — See Reports, p. 39. 



€. Thermopile and Galvanometer combined. By Professor Geoege Forbes. 



7, On the Intensity of Reflection from Glass and other Surfaces. 

 By Lord Ratleigh, D.G.L., LL.D., Sec. R.S. 



8. A Note on some Observations of the Loss which Light suffers in passing 

 through Glass. By Sir John Conrot, Bart., M.A. 



The object of the experiments was to determine the percentage of light which 

 passed through plates of glass of the same kind hut of different thickness. The 

 amount reflected from the first surface would he the same in all cases, and, assimiing, 

 as is usually done, that the same percentage of the incident light is reflected 

 from both surfaces, the amount reflected from the second surface would be nearly 

 the same — the diiference in the amount of light which reached the second surface, 

 owing to the increased absorption in the thicker plates, being but slight. It was 

 therefore thought that this method would aiford a means of determining not only 

 the amount reflected, but also the amount absorbed, without assuming the truth of 

 the formulse for reflection. 



A photometer, similar to that described in Pickering's ' Physical Manipulation ' 

 (pt. i. p. 132), was used for most of the observations ; but measurements were also 

 made with a polarising photometer, and also with another arrangement, in which of 

 two white surfaces illuminated by the same lamp, one was seen directly, and the 

 other through the glass. If atmospheric absorption be neglected, which at such 

 distances is of course insensible, the apparent brightness of an illimiinated surface 

 does not vary with the distance ; no correction was therefore needed with this 

 instrument for the optical shortening of the path of the ray, owing to the refrac- 

 tive power of the glass, which had to be allowed for in the experiments made with 

 the first-mentioned photometer. 



The experiments were made with plates of Messrs. Chance's lighthouse glass, 

 Tarying in thickness from 6'5 to 24 millimetres, and with Field's ordinary dense 

 flint, from 7 to 91'3mm. thick. "Vk'^ith Messrs. Chance's glass the transmitted 

 light ranged from 91'50 to 87-16 per cent., and with the flint-glass from 88-83 to 

 80-74 per cent. 



The experiments are incomplete, and are being continued. 



9. On an Experiment showing that a Divided Electric Current may he 

 greater in both branches than in the Tnains. By Lord Ratleigh, 

 D.C.L., LL.B., Sec. R.S. 



SATURDAY, SEPTEMBER 4. 

 The Section did not meet. 



MONDAY, SEPTEMBER 6, 

 The following Reports and Papers were read : — 



1. Report of the Committee for preparing instructions for the practical 

 work of Tidal Observation. — See Reports, p. 40. 



