246 
sm J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
the incident light, 91‘440 per cent, actually passed through; and with the other, of 
which the index was 1'5225, 91‘155 w'as transmitted, instead of 91'763. 
Vogel published in the ^Berliner Monatsberichte ’ for 1877 (jd. 138) some deter¬ 
minations of the absorption of crown, and of two kinds of flint, glass for different 
parts of the spectrum, but states that, as the three glasses were not ecpially well 
polished, no conclusion can be drawn as to their relative merits. 
M. Allaud (“ L’Intensite des Phares,” ‘ Annales des Pouts et Chaussees,’ 1876, 
p. 31) states that the absorption of light by the glass which it passes through is given 
“ par une formule exponentielle, mais on pent sans grande erreur la supposer propor- 
tionelle a I’epaisseur et I’evaluer a raison de 0'03 par centimetre de verre traverse,” 
but does not give an account of any experiments made to determine the amount 
absorbed. 
Since the experiments described in this paper were commenced Lord Rayleigh has 
published (‘Roy. Soc. Proc.,’ vol. 41, p. 275) an account of some determinations he 
has made of the intensity of the light reflected from glass at a nearly perpendicular 
incidence; he came to the conclusion that “ recently polished glass surfaces have a 
reflecting power differing not more than 1 or 2 per. cent, from that given by Fresnel’s 
formula; but that after some months or years the reflection may fall off from 10 to 
30 per cent., and that without any apparent tarnish.” 
The experiments of which I have the honour of presenting an account to the Royal 
Society were commenced in order to determine, if possible, the amount of light lost by 
transmission tln-ough glass, without assuming the truth of the formulm for reflection, 
and also to determine experimentally the amount reflected from the surface of the 
glass. 
It was thought that one way in which this might be effected would be by taking 
plates of the same kind of glass, but of different thicknesses, and observing the 
amount of light which passed through; the reflection from the first surface would be 
the same in all cases, whilst that from the second would only differ slightly in amount, 
as, owing to the increased absorption of the thicker pieces, less light would reach the 
second surface; but, the absorption being small, and photometric methods not very 
exact, it was thought that this would hardly produce any sensible difference in the 
results. 
It was also hoped that it might be found j^ossible to determine the reflected light 
directly, since, as Lord Rayleigh has pointed out {Joe. cit.) with reference to Professor 
Rood’s experiments, any error in the measured amount of the light transmitted would 
give rise to a very much greater error in the estimated amount of the reflected light. 
This wms subsequently accomplished by measuring the relative intensities of the 
illumination produced by two Argand gas flames, when the light from both fell directly 
on the photometric surfaces, and when the light from one fell directly, whilst that 
from the other reached the photometer after reflection from the surface of a piece of 
glass. 
