PROCTOR, ON LIGHT. 165 
has no part in the reflection; I only wish to show, in a striking 
manner, that it is not the only agent—it is but a partner in the 
firm which does the business. A very good illustration of 
these two propositions is found in the total reflection which we 
observe most conveniently with a prism. The light entering 
by the surface A falls upon B, and is reflected through C. The 
brilliancy of the reflection is intense when air is in contact 
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or 
with the surface B; but if you bring a second prism in con- 
tact with it, as at D, the reflection ceases, and the ray passes 
straight on. If it was the surface of the glass which reflected, 
why not have a greater reflection when you have two surfaces? 
But if it is the air which reflects, the displacing of the air 
by the second glass surface accounts for the reflection ceasing. 
If you place water there, you have a reflection very much 
less than from air, but very perceptible; with turpentime it is 
scarcely visible ; and if mercury is substituted, the reflection, 
though greater than with these other materials, is yet de- 
cidedly inferior to that from air. No reflecting surface with 
which we are acquainted will bear any comparison with the 
brilliance of air, when total reflection from its surface is 
obtained in this way. Let me caution you not to misunder- 
stand me on this point. I do not say that total reflection 
cannot be obtained from anything else but air; but that, in 
practice, it is always obtained from a surface of air in con- 
tact with glass or some other highly refracting substance, 
the glass as well as the air being requisite for its production. 
In quitting the subject of reflection, I may remark that 
the brilliant surfaces of mercury, polished silver, &c., do not 
reflect so much hght as we might suppose from their bright- 
ness ; even common white paper reflects as much. 
To illustrate this I have formed a little conical shade of 
white paper; under it is a convex reflector, formed by silver- 
ing the concave side of a watch-glass, and on the middle of 
