LIGHT. The laws which govern the phenomena of light, when propagated through a vacuum or a uniform uncrystallised medium, form, with the exception of the laws of reflection, the only branch of optics with which the ancients had a scientific acquaintance. The discovery of single and double refraction, of _chromatic dispersion, polarisation, and of mutual interference as exhibited in the various experiments of diffraction, have formed both to the practical and scientific men of modern times the sources of exploration in the grandest phenomena of the physical world, as well as in those which belong to the most delicate scale.
The first branch of this subject has been called photometry, and is confined to light emanating from whatever sources, but unmodified iu its progress through space by any external influences, though strictly speaking photometry relates to the comparison, as to intensity, of two streams of light under all circumstances. [PuoromErEn.] In our cognisance of the form of objects by the sense of feeling, the hand or other part of the body is brought into contact with the object of our perception, and by some ancient philosophers it was supposed that in like manner rays of tight were emitted from the eye in straight hoes, sad by their hupulse on distant Leslies mused our perception of their form and colour. But on examining the structure of the eye (:es) we and that to whatever luminous object we direct the optic axis, an image of the object is depicted on the retina in connection with a system of nerves, in the Name manner that similar pictures are formed by mechanical contrivances, such as lenses upon screens, &c.; hence it is obvious that vision is caused by light proceeding from the obserred object. Now since bodies are perceptible in all positions, it follows that light emanates from luminous bodies in all directions; and as to opaque bodies, the light which falls on the irregularities of their surfaces is in a great measure afterwards scattered in all directions, 11 which they become viable to any number of observers.
Su that a luminous point in enveloped by a spherical surface of a certainnadius, but having that point placed at its centre, the surface will be obviously illuminated all over uniformly with a brightness or intensity depending on the magnitude of the radius. We can ascertain
the connection between this brightness and the radius, by supposing the light from the nurse source diffiused over another and concentric spherical surface of a greater or lees radius; it will evidently be more or lass intensely diffused in the exact proportion in which the one surface is less or greater than the other. Now spherical surfaces arc proportionel to the equates of the radii, therefore the intensity of light proceeding in vacuo from any luminous origin must be in the inverse proportion of the square of the distance from that poiut. We have employed the term proceeding as applicable to light, because that by two independent astronomical phenomena, namely, the aberration of light, and by the retardation of the eclipses of Jupiter's satellites, we are alike taught that light, whether of the sun, planets, satellites, or fixed stars, is not propagated instantaneously throughout apace, but travels with a velocity in round numbers of 192,000 miles per second. \Within the last few years M. Fizeau has succeeded, by a most ingenious contrivance, not only in rendering sensible the time which light takes to travel the distance of a few miles, but even in measuring its velocity by direct experiment. (' Comptes Rendus,' tom. 29, p. 90.) A ray of light has its origin at a luminous point, whence it diverges in an infinitely small solid or conoidal angle, and is the geometrical element of the total spherical emanation of that point. These rays in straight lines in YAM) or a uniform medium, for no opaque screen the luminous point from view, except when placed in the straight line joining the eye of the observer with the origin of the light, or, which is the same, we cannot see through bent tubes ; but the modifications suffered by light at the surfaces of bodies, and in the interior of media, cause generally a deflection, sometimes sudden, at others gradual, in the direction of the ray.