of which the mirror forms part, is called the centre of curvature, the line O L the principal axis. By remembering these very simple definitions, we shall be able to understand the action of these mirrors without the slightest difficulty.
To understand how the rays of light are reflected from the surface of the mirror N M at the point F, which is called the focus, we have only to consider the mirror as consisting of an infinite number of facets, all inclined towards that particular point, and forming by reason of their immense numbers a regular spherical surface. In considering the mirror from this point of view, we can immediately see that, on account of the inclination of the supposed facets, the rays that they receive are all reflected back again at the same point; and it may be proved geometrically, that when the incident rays are parallel the focus will be situated somewhere on the line O C, its position depending on the curvature of the mirror.
If, therefore, we receive on a spherical mirror a pencil of sunlight, the rays which compose it may be regarded as parallel, the sun being at so great a distance from the earth; it follows that these rays will all be reflected together in a particular point, viz., at F, and if any object be placed there it will be illuminated with great brilliancy. The laws governing the reflection of heat being nearly similar to those regulating the action of light, the rays reflected from a burning body will ignite any inflammable substance placed at the point F. The focus for parallel rays is called the principal focus of a mirror. Having described the effects of parallel rays, let us now see what happens when the source of light is close to the mirror. If it is placed at a very small distance, the luminous rays are divergent instead of parallel, and their meeting point becomes changed in accordance with the laws laid down at the beginning of
