PROCTOR, ON LIGHT. 153 
rapidly less from the loss it sustains, by a portion being 
transmitted at each reflection; 10, 11, 12, and 13, indicate 
the portions transmitted through the upper surface of the 
glass from the reflected rays, 3, 5, 7, and 9, respectively ; 
14, 15, 16, and 17, indicate the portions transmitted through 
the lower surface of the glass from the primary ray 2 and 
the rays 4, 6, and 8, respectively ; the ray 14 describes a 
path in B similar to that described in A, by the other portion 
of the primary ray, but at its second reflection it is jomed 
by the ray 15, and at its fourth by the ray 16. Its second 
reflection is accompanied by a transmission of a portion 
through the upper surface of B, which then re-enters A, 
undergoing again the same series of transmissions, reflections, 
and passages from one plate to the other, an indefinite, if 
not an infinite number of times; and if, stead of two plates 
there were twenty, the same would go on in a much more 
complicated series. 
Only a small portion of light finding its way through the 
whole series of plates, and all the remainder being reflected or 
absorbed during its many passages backward and forward, 
the proportion of light reflected at each surface, compared 
with that which is transmitted at the same, depends upon the 
amount of refraction which the transmitted portion under- 
goes; and the amount of refraction depends upon the angle 
of incidence, and the comparative densities of the glass and 
air, or other media with which we are dealing. If instead of 
air between the plates we have water, there will be less re- 
fraction at each surface, because of the less difference in the 
densities in the two media; consequently, there will also be 
more transmission and less reflection ; the bundle of plates 
will become, as a whole, more transparent. 
We have here three bundles of glass plates; in one there 
is air intervening, and you will observe it has considerable 
opacity, and reflects much light ; in the second there is water 
between the plates, and it is more transparent, and reflects 
less light ; the third, in which spirit of turpentine replaces 
the air, is so transparent, and reflects so little light from the 
interior plates, that we might almost suppose it a solid block 
of glass. I have been speaking of the action of the surfaces, 
but every plate, however thin, consists of something more than 
two surfaces, and the substance which intervenes exerts what 
is called an absorbing power over the light as it passes 
through it. Though I do not think absorption a correct 
expression of what takes place, I will submit to the conven- 
tionality, and use it till something more correct is adopted. 
Supposing the bundles of plates to consist of a countless num- 
