280 
SIR J. CONROY ON THE AMOUNT OE LIGHT REFLECTED 
Part IL 
When light passes throngli a transparent plate it is diminished by reflection at the 
two surfaces, and by “ obstruction ” v.dthin the plate, the cause of obstruction being 
that a part of the light which has entered the plate is absorbed and, unless the plate 
be absolutely homogeneous, a part scattered. 
If r be the ratio of the light reflected by the first surface, and r' by the second 
surface, to the light incident upon them, a the coefficient of transmission, and t the 
thickness of the plate, then the intensity of the transmitted beam is given by the 
expression i = lpp'a\ where p = {I — r) and p' = (l — r'). 
I, ^, and t being known, by eliminating pp', a can be readily calculated. The value 
of i depends in the case of coloured media on the refrangibihty of the light, but in the 
case of the two kinds of glass used in these experiments it may be taken to be the 
same for light of all wave-lengths. 
Table XVI. contains the values of a for a thickness of one millimetre of crown and 
flint glass, obtained by combining in pairs the five values of i for crown glass, and the 
four values for flint glass, contained in Table VI. 
Table XVI. 
Values of a. 
Crown glass. 
Flint glass. 
•99685 
•99906 
■99690 
•99884 
•99744 
•99887 
•99729 
■99893 
•99692 
•99837 
•99752 ! 
•99897 
•99750 ! 
•99809 ' 
Mean ■99884 
•99763 ; 
•99733 ; 
Mean ^99735 
Dr. Robinson, in the joaper already mentioned (‘Phil. Trans.,’ 18G9, p. 160), gives 
the values of n in the expression i = Ip^e~"*', p^ being calculated from Fresnel’s 
formula, and t being the thickness in inches. From the values given by Dr. Robinson 
for a cylinder of crown, and a prism of flint, glass, both of Messrs. Chance’s manu¬ 
facture, the values of the coefficient « were calculated for a thickness of one millimetre. 
