CATOPTRICS is that part of optics which explains the properties of reflected light, and particularly that which is reflected from mirrors.

As this and the other branches of Optics are fully treated under the collective word, we shall, in the present article, 1st, just give a summary of the principles of the branch, in a few plain aphorisms, with some preliminary definitions; and, 2ndly, infer a set of entertaining experiments founded upon them.

SECT. I. Definitions.

Definitions. 1. Every polished body that reflects the rays of light is called a mirror, whether its surface be plane, spherical, conical, cylindric, or of any other form whatever.

Plate xxii. 2. Of mirrors there are three principally used in optical experiments, which are, the plane mirror, GHI, (fig. 3.) the spherical convex mirror, GHI, (fig. 4.) and the spherical concave mirror, GHI, (fig. 5.)

3. The point K, (fig. 4, 5.) round which the reflecting surface of a spherical mirror is described, is called its centre. The line KH, drawn from its center perpendicular to its two surfaces, is the axis of the mirror, and the point H, to which that line is drawn, is its vertex.

4. The distance between the lines AG and BG, (fig. 3.) is called the angle of incidence, and the distance between BG and CG is the angle of reflection.

SECT. II. Aphorisms.

1. The image DF, (fig. 3.) will appear as far behind the mirror, as the object AC is before it.

2. The image will appear of the same size, and in the same position as the object.

3. Every such mirror will reflect the image of an object of twice its own length and breadth.

4. If the object be an opaque body, and its rays fall on the mirror nearly in direct lines, there will be only one image visible, which will be reflected by the inner surface of the glass.

5. If the object be a luminous body, and its rays fall very obliquely on the mirror, there will appear, to an eye placed in a proper position, several images; the first of which, reflected from the outer surface of the glass, will not be so bright as the second, reflected from the inner surface. The following images, that are produced by the repeated reflections of the rays between the two surfaces of the glass, will be in proportion less vivid, to the eighth or tenth, which will be scarce visible.

1. The image DF, (fig. 4.) will always appear behind it.

2. The image will be in the same position as the object.

3. It will be less than the object.

4. It will be curved, but not, as the mirror, spherical.

5. Parallel rays falling on this mirror will have the focus or image at half the distance of the center K, from the mirror.

6. In converging rays, the distance of the object must be equal to half the distance of the center, to make the image appear behind the mirror.

7. Diverging rays will have their image at less than half the distance of the center. If the object be placed in the center of the mirror, its image will appear at one-eighth of that distance behind it.

1. That point where the image appears of the same dimensions as the object, is the center of that mirror.

2. Parallel rays will have their focus at one half the distance of the center.

3. Converging rays will form an image before the mirror.

4. In diverging rays, if the object be at less than one half the distance of the center, the image will be behind.