44 
MR. FARADAY ON THE 
DO. As there is a certain quantity of light intercepted by glass which is alto- 
gether dependent upon and in proportion to its colour, it is evident that this 
property of the heavy glasses must be considered in relation to their use in 
telescopes ; but there appears no reason for supposing they will ultimately 
prove inapplicable on this account. The colour of the glass already obtained 
is far less in depth than that of the crown glass constantly used in the con- 
struction of telescopes, which yet intercepts by its colour no important quan- 
tity of light ; and if two plates 8 or 10 inches long, one of the yellow heavy 
glass and the other of crown glass, be looked through edgeways, it will be 
seen in a moment that the crown glass intercepts by far the most light. The 
colour of the glass is of no consequence, otherwise than as causing a loss of 
light from interception ; for the tinge which is cast over objects looked at 
through a telescope constructed with it is scarcely perceptible to the most 
acute eye, and quite unimportant. When to these circumstances is added the 
reasonable expectation entertained of removing a large proportion of the little 
remaining colour by the use of purified silica (21), it will not be anticipated 
that experience will prove the glass faulty in this respect. 
100. There is one veiy important action of the glass upon light, however, 
which may perhaps interfere more with its application, in telescopes at least, than 
any other, i. e. its reflective power. This is very strong in all the heavy glasses, 
far stronger than in flint, and exceedingly surpassing the similar power of 
crown glass. It is in proportion, as might have been expected, to the refractive 
power and the density of the glasses, all these properties increasing with the 
oxide of lead. The loss of light occasioned by the reflection from the two sur- 
faces of a plate through which a ray is passed, appears to me to be greater 
than from the united action of both colour and bubbles in a piece of glass 
7 inches thick. 
I endeavoured to ascertain the comparative quantities of light reflected by 
these heavy and other glasses, in some photometrical experiments made upon 
the principle of similar shadows, measuring only the reflexion from the first 
surface of the different glasses, that from the second surface being destroyed. 
The ray was made incident in all the cases at an angle of 45°. It was obtained 
from a small single-wicked lamp a ; and when reflected, its intensity was 
measured by the distance of a similar lamp b, whose direct light cast the 
