Mrs. B. Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that shine are not luminous; for a luminous body is one that shines by its own light, as the sun, the fire, a candle, &c.

Emily. Polished metal then, when it shines with so much brilliancy, is not a luminous body ?

Mrs. B. No, for it would be dark if it did not receive light from a luminous body; it belongs, therefore, to the class of opaque or dark bodies, which comprehend all such as are neither luminous nor will admit the light to pass through them.

Emily. And transparent bodies, are those which admit the light to pass through them; such as glass and water ?

Mrs. B. You are right. Transparent or pellucid bodies, are frequently called mediums; and the rays of light which pass through them, are said to be transmitted by them.

Light, when emanated from the sun, or any other luminous body, is projected forwards in straight lines in every possible direction; so that the luminous body is not only the general centre from whence all the rays proceed; but every point of it may be considered as a centre which radiates light in every direction. (fig. 1. plate XV.)

Emily. But do not the rays which are projected in different directions, and cross each other, interfere and impede each other's course?

Mrs. B. Not at all. The particles of light are so extremely minute, that they are never known to interfere with each other. A ray of light is a single line of

light projected from a luminous body; and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. 2.

Caroline. Is light then a substance composed of particles like other bodies?

Mrs. B. This is a disputed point upon which I cannot pretend to decide. In some respects, light is obedient to the laws which govern bodies; in others it appears to be independent of them: thus though its course is guided by the laws of motion, it does not seem to be influenced by those of gravity. It has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point; but we are so ignorant of the intimate nature of light, that an attempt to investigate it would lead us into a labyrinth of perplexity, if not of error; we shall therefore confine our attention to those properties of light which are well ascertained.

Let us return to the examination of the effects of the radiation of light from a luminous body. Since the rays of light are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot move in a curve line round the body.

Caroline. No, certainly; for it would require some other force besides that of projection, to produce motion in a curve line.

Mrs. B. The interruption of the rays of light, by the opaque body, produces, therefore, darkness on the opposite side of it; and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadow.

Emily. A shadow then is nothing more than darkness produced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind the opaque body.

Caroline. Why then are shadows of different degrees of darkness; for I should have supposed from your definition of a shadow, that it would have been perfectly black ?

Mrs. B. It frequently happens that a shadow is produced by an opaque body interrupting the course of the rays from one luminous body, while light from another reaches the space where the shadow is formed, in which case the shadow is proportionally fainter. This happens if the opaque body be lighted by two candles if you extinguish one of them, the shadow will be both deeper and more distinct.

Caroline. But yet it will not be perfectly dark.

Mrs. B. Because it is still slightly illumined by light reflected from the walls of the room, and other surrounding objects.

You must observe, also, that when a shadow is produced by the interruption of rays from a single luminous body, the darkness is proportional to the intensity of the light.

Emily. I should have supposed the contrary; for as the light reflected from surrounding objects on the shadow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined.

Mrs. B. Your remark is perfectly just; but as we have no means of estimating the degrees of light and of darkness but by comparison, the strongest light will