Mrs. B. Yes. Among the variety of tones, there are some which, sounded together, please the ear, producing what we call harmony, or concord. This arises from the agreement of the vibrations of the two sonorous bodies; so that some of the vibrations of each strike upon the ear at the same time. Thus, if the vibrations of two strings are performed in equal times, the same tone is produced by both, and they are said to be in

unison.

Emily. Now, then, I understand why, when I tune my harp in unison with the piano-forte, I draw the strings tighter if it is too low, or loosen them if it is at too high a pitch: it is in order to bring them to vibrate, in equal times, with the strings of the pianoforte.

Mrs. B. But concord, you know, is not confined to unison; for two different tones harmonize in a variety of cases. If the vibrations of one string (or sonorous body whatever) vibrate in double the time of another, the second vibration of the latter will strike upon the ear at the same instant as the first vibration of the former; and this is the concord of an octave.

If the vibrations of two strings are as two to three, the second vibration of the first corresponds with the third vibration of the latter, producing the harmony called a fifth.

Caroline. So, then, when I strike the key-note with its fifth, I hear every second vibration of one, and eve. ry third of the other at the same time?

Mrs. B. Yes; and the key-note struck with the fourth is likewise a concord, because the vibrations are as three to four. The vibrations of a major third with the key-note, are as four to five; and those of a minor third, as five to six.

There are other tones, which, though they cannot be struck together without producing discord, if struck successively, give us the pleasure which is called melody. Upon these general principles the science of mu

sic is founded; but I am not sufficiently acquainted with it to enter any further into it.

We shall now, therefore, take leave of the subject of sound; and, at our next interview, enter upon that of optics, in which we shall consider the nature of vision, light, and colours.

CONVERSATION XIV.

ON OPTICS.

OF LUMINOUS, TRANSPARENT, AND OPAQUE BODIES.-OF THE RADIATION OF LIGHT.OF SHADOWS. OF THE REFLECTION OF LIGHT.-OPAQUE BODIES SEEN ONLY BY REFLECTED LIGHT.-VISION EXPLAINED.-CAMERA OBSCURA.-IMAGE OF OBJECTS ON THE RETINA.

CAROLINE.

I LONG to begin our lesson to day, Mrs. B., for I expect that it will be very entertaining.

Mrs. B. Optics is certainly one of the most interesting branches of Natural Philosophy, but not one of the easiest to understand; I must therefore beg that you will give me the whole of your attention.

I shall first inquire, whether you comprehend the meaning of a luminous body, an opaque body, and a transparent body.

Caroline. A luminous body is one that shines; an opaque

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,