282 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
The observed values of the light reflected by the crown glass do not agree quite so 
well as those for the flint glass ; this may be due to the fact that all the measurements 
with the flint glass were made with one thick block, whilst several pieces of crown 
glass were used for the transmission experiments, and these plates may have differed 
slightly both in their composition and in the polish of their surfaces. Making due 
allowance for this, and for the approximate character of all photometric measurements, 
the agreement between the results obtained by two entmely distinct methods is, 
probabl}', quite as close as could have been anticipated. 
The calculated value for both kinds of glass considerably exceeds the observed. As 
has already been mentioned, the refractive indices, as determined with the large 
prisms used for the reflection experiments, and with the small prisms, differed slightly; 
the differences, however, — ’0008 and + ’0055, are not sufficiently great to affect the 
result to any considerable extent. Thus, at a perpendicular incidence the two 
theoretical values for the reflected light are 4T87 and 4'176 for the crown glass, and 
5'780 and 5'856 for the flint glass, or a difference of O’Oll and 0'076 per cent, of the 
incident light, a quantity which is, of course, quite inappreciable photometrically. 
Both the crown glass and the flint glass had been recently polished, the former by 
Messrs. Chance and the latter by Mr. Hilger, both kinds of glass having been 
ground with emery and polished with rouge; the crown glass vms partially machine- 
polished but finished by hand, the flint glass entirely hand-polished. The glass 
surfaces were always well cleaned with wash-leather immediately before being used, 
as after being left in the laboratory for some days they were usually somewhat 
tarnished; the films, however, were easily removed, and in no case could any 
deterioration of the surface be detected. 
The effect of repolishing the glass was to increase its reflective power, but Tables X. 
and XL show that the two kinds of glass behaved somewhat differently. Immediately 
after repolishing both reflected more than the theoretical amount of light; but, whilst 
the crown continued to do so, the reflective power of the flint decreased, and after an 
interval it reflected the theoretical amount. 
Table XIX. 
Crown gla«B. 
Flint glas3. 
PerceDtage of light reflected before repolishing 
378 
5-20 
?? 9? 
after repolishing 
r Immediately .... 
t After an interval 
4-27 
4-31 
6T4 
572 
5? ?? 
calculated from 
the observed index of 
refraction . 
4T9 
578 
A number of measurements were made of the light reflected by the crown glass at 
various angles before and after I’epolishing; the means of the results contained in 
