them, vision was performed by something which emanated from the eye to the object; and the sense of Sight was explained by the analogy of that of Touch. In this view, then, the sensation was represented as independent of the nature of the body seen; and all objects should be visible, whether in the presence of a luminous body or not. This strange hypothesis held its ground for many centuries. The Arabian astronomer, Alhazen, who lived in the latter part of the eleventh century, seems to have been the first to refute it, and to prove that the rays which constituted vision came from the object to the eye.

(3) The light of a luminous body emanates from it in all directions. Thus, the light of a lamp or candle is seen in all parts of a room, if nothing intervenes to intercept it; and the light of the Sun illuminates the Earth, the Planets, and their satellites, in whatever position they may be placed respecting it.

Each physical point of a luminous body is an independent source of light, and is called a luminous point.

(4) Non-luminous bodies are distinguished into two classes, according as they allow the light which falls upon them to pass freely through their substance, or intercept it. Bodies of the former kind are said to be transparent; those of the latter, opaque.

There are no bodies in nature actually corresponding to these extremes. The most transparent bodies, as air and wetintercept a sensible quantity of light, when of sufficient thickness; and, on the other hand, the most opaque bodies, such as the metals, allow a portion of light to pass through their substance, when reduced to laminæ of exceeding tenuity.

(5) In the same homogeneous medium, light is propagated in right lines, whether it emanates directly from luminous. bodies, or is reflected from such as are non-luminous.

n

b

This is proved by the fact that when an opaque body is interposed in the right line connecting the eye and the luminous source, the light of the latter is intercepted, and it ceases to be visible. The same thing is proved also by the shadows of bodies, which, when received upon plane surfaces perpendicular to the path of the light, are observed to be similar to the section of the body which produces them.

This property of light was recognised by the ancients; and by means of it the few optical laws which were known to them became capable of mathematical expression and reasoning. Any one of these lines, proceeding from a luminous point, is called in optics a ray.

(6) In a perfectly transparent medium, the intensity of the light proceeding from a luminous point varies inversely as the square of the distance.

This is easily proved, if light be supposed to be a material emanation of any kind. For the intensity of the light, received upon any spherical surface whose centre is the luminous point," is as the quantity of the light directly, and inversely as the space over which it is diffused. But none of the light being lost, the quantity of light received upon any spherical surface is the same as that emitted, and is therefore constant; and the space of diffusion, or the area of the spherical surface, is as the square of its radius. Hence the intensity of the light is inversely as the square of the radius, i. e. inversely as the square of the distance.

Let the light be supposed to emanate from the points of an uniformly luminous surface, which we shall suppose to be a small portion of a sphere. Then the quantity of light emitted is proportional to the quantity emitted by a single point, and the number of points (or area) conjointly. Hence if a denote the area of the luminous surface, and i the quantity emitted from a single point, which is a measure of the

absolute brightness, the intensity of the illumination, at any distance d, is

ai Ja

(7) A plane surface, whose dimensions are small in comparison with the distance, and which is perpendicular to the incident light, may, without sensible error, be considered as a portion of a spherical surface concentric with the luminary. The intensity of the illumination, therefore, or the quantity of light received upon a given portion of such a plane, is expressed by the formula of the preceding Article.

When the surface is inclined to the incident light, the quantity of the light received by any given portion is diminished in the ratio of unity to the sine of the angle of inclination. The intensity of the illumination is, therefore, diminished in the same proportion, and is expressed by the formula

ai sin 0
d2

O being the inclination of the surface to the incident light.

(8) Experience proves that the eye is incapable of comparing directly two lights, so as to determine their relative intensity. But, although unable to estimate degrees, the eye can detect differences of intensity with much precision; and with this power it is enabled (by the help of the principles just established) to compare the intensities of two lights indirectly.

Let two portions of the same paper (or any similar reflect ing surface) be so disposed, that one of them shall be illuminated by one of the lights to be compared, and the other by the other, the light being incident upon each at the same angle. Then let the distance of one of the lights be altered, until there is no longer any appreciable difference in the inten

1

sities of the illuminated portions. The illuminating powers of the two lights will then be as the squares of their respective distances; and their absolute brightnesses as the illuminating powers directly, and as their luminous surfaces inversely. For, if i and i' denote the absolute brightnesses of the two lights, a and a' the areas of the luminous surfaces, and d and d' their distances from the paper, the intensities of illumination are ai sin

d2

and

a'ï' sin 0

d'2

respectively; and these being rendered

The following simple and convenient mode of practising this method was suggested by Count Rumford. A small opaque cylinder is interposed between the lights to be compared and a screen; in this case it is obvious that each of the lights will cast a shadow, which is illuminated by the other light, while the remainder of the screen is illuminated by both lights conjointly. If, then, one of the lights be moved, until the shadows appear of equal intensity, their illuminations are equal, and, therefore, the illuminating powers of the two lights are to one another as the squares of their distances from the

screen.

(9) Light is propagated with a finite velocity.

This important discovery was made in the year 1676, by the Danish astronomer, Olaus Roemer. Roemer observed that when Jupiter was in opposition, and therefore nearest to the Earth, the eclipses happened earlier than they should according to the astronomical tables; while, when Jupiter was in conjunction, and therefore farthest, they happened later. He thence inferred that light was propagated with a finite velocity, and that the difference between the computed and observed times was due to the change of distance. This difference is found

to be 8m 13s; and accordingly the velocity of light is such, that it traverses 192,500 miles in a second of time.

(10) The velocity of light, combined with that of the Earth in its orbit, was afterwards applied by Bradley to explain the phenomenon of the aberration of the fixed stars. From the theory of aberration so explained, it followed that the velocity of the light of the fixed stars is to the velocity of the Earth in its orbit, as radius to the sine of the maximum aberration. This latter quantity-the constant of aberration, as it is called

-is now found to be 20′′-36; and the Earth's velocity being known, the velocity of the light of the fixed stars is deduced. The value so obtained is 191,500 miles in a second, which differs from that inferred from the eclipses of Jupiter's satellites, by only the th part of the whole.

200

From this it follows, that the direct light of the fixed stars, and the reflected light of the satellites, travel with the same velocity.

(11) The velocity of light, emanating from a terrestrial source, has been recently measured by M. Fizeau, by direct experiment. The first idea of this experiment was communicated to M. Arago, by the Abbè Laborde, a few years before; its principle will be understood from the following description.

Let the light of a lamp be reflected nearly perpendicularly by a mirror placed at a considerable distance; let a toothed wheel, the breadth of whose teeth is equal to that of the interval between them, be interposed near the luminous source; and let the mirror be so adjusted that the light passing through one of these intervals is reflected to that diametrically opposite. If the eye be placed behind the latter interval, the wheel being at rest, it will perceive the reflected ray, which has traversed a space equal to double the distance of the mirror from the wheel. But if, on the other hand, the wheel be made to revolve rapidly, its velocity may be such that the light transmitted through the opening at one extremity of the diameter