AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
281 
Index for line E. 
n. 
ct* 
Crown glass. 
1-.5200 
0-0272 
0-99893 
Flint glass. 
1-6216 
0-0218 
0-99914 
From the values of a it would appear that these two specimens of glass absorbed 
somewhat less light than those used in the experiments of which an account is 
contained in this paper, but Dr. Robinson’s results depend on the value of p being 
“ calculated accurately from Fresnel’s formula,” and if, as seems probable, glass 
usually reflects less than the theoretical amount of light, the amount absorbed would 
necessarily appear less than it really was. 
From the mean values of a given in Table XVI. the values of p (on the assumption 
that p = p) were obtained by calculating the values of V for the different thicknesses 
of the two kinds of glass used in these experiments and then introducing these values 
into the equation i = Ipp'a*. 
Table XVII. 
Values of p. 
Crown glass. 
Flint glass. 
6-5 mm. plate. -9648 
11-5 „ -9636 
15-0 „ -9629 
18-5 „ -9648 
24-3 „ -9642 
Mean . . -9641 
7-0 mm. plate. -9463 
490 „ -9508 
69-5 „ -9461 
91-3 „ -9475 
Mean . . -9477 
The value of r for the crown glass is therefore ‘0359, and for the flint glass ‘0523. 
The amount of light which according to theory should have been reflected by the 
glass was calcidated by the expression , where n is the index of refraction. 
These values, and also the amount of the reflected light, as determined directly (see 
Tables VIII. and IX.), are given in Table XVIIL 
Table XVIII. 
Percentage amount of Light Pteflected 
Ob.served. 
By transmission. 
By reflexion. 
Calculated. 
Crown glas.s .... 
3-59 
3-78 
4-187 
Flint glass .... 
5-23 
5-20 
5-780 
MDCCCLXXXIX. —A. 
