MRS. B.
The effect would in that case be so trifling, at the distance at which the bullet and the thermometer are from each other, that it would be almost imperceptible. The mirrors, you know, greatly increase the effect, by collecting a large quantity of rays into a focus; place your hand in the focus of the mirror, and you will find it much hotter there than when you remove it nearer to the bullet.
EMILY.
That is very true; it appears extremely singular to feel the heat diminish in approaching the body from which it proceeds.
CAROLINE.
And the mirror which produces so much heat, by converging the rays, is itself quite cold.
MRS. B.
The same number of rays that are dispersed over the surface of the mirror are collected by it into the focus; but, if you consider how large a surface the mirror presents to the rays, and, consequently, how much they are diffused in comparison to what they are at the focus, which is little more than a point, I think you can no longer wonder that the focus should be so much hotter than the mirror.
The principal use of the mirrors in this experiment is, to prove that the calorific emanation is reflected in the same manner as light.
CAROLINE.