Emily. But each set of these irregular vibrations, if repeated at equal intervals, would, I suppose, produce a musical tone? It is only their irregular succession which makes them interfere, and occasions discord.

Mrs. B. Certainly. The quicker a sonorous body vibrates, the more acute, or sharp, is the sound produced.

Caroline. But if I strike any one note of the pianoforte repeatedly, whether quickly or slowly, it always gives the same tone.

Mrs. B. Because the vibrations of the same string, at the same degree of tension, are always of a similar duration. The quickness or slowness of the vibrations relate to the single tones, not to the various sounds which they may compose by succeeding each other. Striking the note in quick succession, produces a more frequent repetition of the tone, but does not increase the velocity of the vibrations of the string.

The duration of the vibrations of strings or chords depends upon their length, their thickness or weight, and their degree of tension: thus, you find, the low bass notes are produced by long, thick, loose strings; and the high treble notes by short, small, and tight strings.

Caroline. Then the different length and size of the strings of musical instruments, serves to vary the duration of the vibrations, and consequently, the acuteness of gravity of the notes ?

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 sono

rous 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 piano-forte.

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 every 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, gives us the pleasure which is called

melody. Upon these general principles the science of music 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 colors.

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...