53

CHAP. III.

NATURAL PHILOSOPHY,

Continued.

OPTICS, Principles of -- Nature of Light -- Refraction of the Rays of Light --Reflexion of Light -Different Refrangibility of the Rays of Light The Rainbow Vision and Structure of the Eye-Optical Instruments · Microscopes — Telescopes ·Camera Obscura --Magic Lantern--Phantasmagoria--Writers on Optics.

THE science of Optics so important to the purposes of life, is a mixed mathematical science; has for its object the investigation of the mechanical properties of light, the laws of the phenomena it exhibits, the application of these laws to practical purposes in the construction of instruments calculated to improve our vision. It enables us to understand the manner in which vision is performed in the eye; it traces the several modifications or alterations which the rays of light undergo in the different parts of that organ; and shews why objects appear, under different circumstances, of different magnitudes, sometimes more distinct, sometimes confused, sometimes nearer, and sometimes more remote. In this comprehensive sense, the science of Optics is considered by Sir Isaac Newton. The history of the science has been detailed by the illustrious Priestley, in a large quarto volume, which has generally been considered as one of the most interesting of his numerous works. In a small compass, such as the nature of this work would admit, it would be scarcely possible to

include a sketch even, of the history, that would be intelligible. We shall, therefore, proceed to explain merely some of the fundamental principles of the science, such as the nature of light, the laws of refraction and reflexion, the nature of vision, and the structure of some of the commoner and more useful instruments.

Of the Nature of Light. It is generally conceived, though the subject does not admit of demonstration, that light consists of inconceivably small particles, flowing with amazing velocity, in all directions, from the luminous or radiant body. This theory of light appears the most simple of any, and serves to explain all the phenomena of vision; and, therefore, has, by the majority of writers on the subject, been assumed as true.

The velocity with which light moves, was first observed by M. Roemer, who ascertained that it travelled from the sun to the earth, a space of 95,000,000 of miles, in about eight minutes, that is at the rate of about 200,000 miles in a second of time. This fact was inferred from the following circumstance: the eclipses of Jupiter's satellites, happen sometimes sooner and sometimes later than the times given by the tables, according as the earth is nearer to, or farther from that planet. Thus, when the earth is at C, Pl. III. fig. 1, between the Sun and Jupiter, his satellites are seen eclipsed about eight minutes sooner than they would be, according to the calculated time, which is given for the mean distance of the planet; but when the earth is in the opposite point of the orbit, D, these eclipses happen eight minutes later than the same tables predict them. Hence it was inferred that the motion of light is not instantaneous, but takes about sixteen minutes to pass over a space, equal to the diameter of the earth's orbit, which is about one hundred and ninety millions of miles in length. If, therefore, the sun were to be annihilated, at any one instant, we should see him, as we now see him, eight minutes after that event happened. Hence, it is easy to calculate how long light is travelling to us from the moon, the other

planets, and even from the fixed stars, if their distances could be ascertained. The distances of the latter are, indeed, immensely great, so that from the nearest of them, suppose Sirius, the dog-star, light must take years even to travel to the earth; and it has been conjectured by some philosophers, that there are stars so remotely situated with respect to the solar system, that the light flowing from them, ever since the creation, and travelling at the rate of 200,000 miles per second, has not even yet reached the earth.

Since the velocity of light is so great, it is justly inferred, that its particles must be almost infinitely small, or else the organs of vision would be destroyed by their impulse upon them. The velocity and minuteness of these particles are not more a matter of wonder, than the rarity of the fluid; for its rays appear to cross each other in all possible directions, without the least apparent disturbance. Thus, we can see through a very small pin-hole, in a piece of paper, a great variety of objects at the same time. Now the light proceeding from these objects must pass at the same instant through the hole, in a great variety of directions, before they arrive at the eye, yet the vision is not in the least disturbed by it.

Again, if a lighted candle be set, in a dark night, upon an eminence, it may be seen all round to the distance of half a mile; so that there is no place within the sphere of a mile in diameter, in which the eye can be placed, where it will not receive some rays from the flame of this candle.

Another circumstance respecting the rays of light is, that they move always in straight lines, as is evident by the impossibility of seeing through a crooked tube.

As light proceeds from a centre, its intensity decreases as the square of the distance from the luminous body increases; that is, at twice the distance from the luminous body, an object will be enlightened only one-fourth as much as it was before; and at three times the distance, only oneninth as much, and so on.

By a ray of light is meant the motion of a single succession of particles in the same line of direction, and this motion is represented by a straight line.

Any parcel of rays proceeding from a point, is called a pencil of rays. By a medium is meant any pellucid or transparent body, which suffers light to pass through it. Thus air, water, and glass, are media.

Of Refraction. Where rays of light, after passing through one medium, on entering another medium of dif ferent density, are bent out of their former course, and change their direction, they are said to be refracted. Thus Sa, (Fig. 1.* Plate III.) is a ray which, when it enters the medium AB, instead of proceeding in the same direction, a n, it is made to move in the direction a x.

If the rays of light, after passing through a medium, enter another of a different density, perpendicular to its surface, they proceed through this medium in the same direction as before, thus the ray represented by Pa, proceeds to b, in the same direction. But if the ray enter obliquely to the surface of a medium, either denser or rarer than that in which it moved before, it is made to change its direction, in passing through that medium. Of this sort of refraction there are two cases.

1. If the medium, which the ray enters, be denser, it moves through it in a direction nearer to the perpendicular drawn to its surface. Thus, Sa, supposed to be in air, upon entering the denser medium, A B, glass, or water, instead of proceeding in the same direction, is bent into the direction, ax, which makes a less angle with the perpendicular, Pb.

2. When a ray of light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. Thus, if xa were were a ray of light passing through glass or water, A B, it will, on arriving at the rarer medium, move in the direction a S, which makes a greater angle with the perpendicular. Refraction is greater or less, that is, the rays are more or less bent or turned

aside from their course, according as the second medium, through which they pass, is more or less dense than the first. Thus light is more refracted in passing from air to glass, than from air to water, because glass is two or three times denser than water.

The refraction of light is thus shown; take an empty basin into a dark room, make a small hole in the windowshutter, so that a ray of light may proceed to the bottom at a given point: mark this spot; then, without disturbing any thing, pour water into the basin, and the ray, instead of proceeding to the point marked, will be bent out of its first direction, and be found at another point nearer the side.

In repeating the experiment, if a piece of looking-glass be laid at the bottom of the basin, the light will be reflected from it; and will be observed to suffer the same kind and degree of refraction, in going out, as in coming in, only in a contrary direction.

If a few drops of milk be put into the water so as to take away its transparency, or if dust be raised in the room by sweeping a carpet, &c., the rays will be rendered much more visible. Another experiment is shown on this subject, and may be repeated very easily: put a shilling into an upright basin or pan when empty, and let a person who is to observe the experiment walk backward, till he just lose sight of the money behind the side of the vessel. Now pour water into the basin, and the observer will see the shilling most distinctly, though neither he nor it has been removed from their places.

Parallel rays of light are such as move always at the same distance from each other: such are those represented at a b, &c., fig. 2. Now if these fall upon a glass planoconvex lens, that is, a lens, one of whose sides is flat, and the other convex, they will be so refracted as to unite in a point, f, behind it, called the focus, the distance of which from the centre of the glass, is called the focal distance, which is nearly equal to the diameter, or to twice the radius of the sphere, from which the lens is supposed to be cut.