Both heat and light behave in the same way in passing from one transparent substance to another, e.g. from air into glass. This can be readily shown by forming images of sources of heat and of light by means of a convex lens, as in the diagram (Fig. 2).
FIG. 2.
The source of light is represented as an electric light bulb, and two of the rays going to form the image of the point of the bulb are represented by the dotted lines. The image is also dotted and can be received on a screen placed in that position.
If now the electric light bulb be replaced by a heated ball or some other source of heat, we find by using a blackened thermometer bulb again that the rays of heat are brought to a focus at almost the same position as the rays of light.
The points of similarity between radiant heat and light might be multiplied indefinitely, but as a number of them will appear in the course of the book these few fundamental ones will suffice at this point.
The Corpuscular Theory.—A little over a century ago everyone believed light to consist of almost inconceivably small particles or corpuscles shooting out at enormous speed from every luminous surface and causing the sensation of sight when impinging on the retina. This was the corpuscular theory. It readily explains why light travels in straight lines in a homogeneous medium, and it can be made to explain reflection and refraction.