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, serve 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 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 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, give 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.

751. How is that species of harmony, called a fifth, produced?

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.

* When music is made by the use of strings, the air is struck by the body, and the sound is excited by the vibrations: when it is made by pipes, the body is struck by the air; but as action and reaction are equal, the effect is the same in both cases,

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, 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

* The direct light of the sun is calculated to be equal to that of 6560 candles, placed at the distance of one foot from the object; and that of the moon, to the light of one candle at 71⁄2 feet distance; of Jupiter at 1620 feet, and of Venus at 421 feet. Sir Isaac Newton supposed rays of light to consist of exceedingly small particles, infinitely smaller than sand, moving from luminous bodies; but later writers suppose them to consist of the undulations of an elastic medium, which fills all space, and which produces the sensation of light to the eye, just as the vibrations of the air produce the sensation of sound to the ear.

752. What is the science called that treats of vision? - 753. What is a luminous body?-754. To what is the direct light of the sun calculated to be equal?-755. To what is the light of the moon-of Jupiter-and of Venus, respectively calculated to be equal?-756. What was Sir Isaac Newton's opinion concerning the nature of light? -757. What is a modern opinion? 758. What are opaque bodies? - 759. What are transparent bodies?-760. What are transparent bodies frequently called?

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 Since the rays

radiation of light from a luminous body. of light are projected in straight lines, when they meet

761. In what manner is light produced from luminous bodies? -762. What is the reason that the progress of rays of light is not impeded by crossing each other? -763. What is a ray of light?-764. What is a pencil of rays?-765. Is light a substance composed of particles of matter like other bodies?-766. In what respect is it subject to the laws of matter?-767. In what respect is it not subject to the laws of matter?-768. What is the consequence when rays of light fall upon an opaque body?

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 pro duced 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.

769. What does this interruption produce in regard to the body?-770. What is a shadow? - 771. Why are shadows of different degrees of darkness? - 772. When a shadow is pro duced by the interruption of rays of light from a single opaque body, to what is the darkness of the shadow proportional?