The science which treats of the nature of light, and the phenomena of vision. Our prescribed limits will not allow of our giving more than a brief outline of the elements of this sublime science, which has employed the pens of some of the most illustrious philosophers in successive ages, whose works upon it are both elaborate and numerous. For the larger portion of the matter on this most interesting branch of natural philsophy, we are greatly indebted to Mr. A. Pritchard, and other modern authors of eminence.
The natural progress of the rays of light is in straight lines; yet, like all other matter, light is influenced by attraction, which sometimes turns it out of its direct course; this happens when it passes out of one medium into another of different density, as from air into water or glass, or from water or glass into air. This disposition or capability of light to be bent, is called its refrangibility; and the change of direction actually assumed, when the rays enter another medium, is called refraction. A very easy experiment will convince any one that light is influenced by some peculiar law when entering or leaving one medium for another. Put one end of a stick into water, and it will appear as if it were broken. This effect is owing to the rays of light being attracted or drawn out of their direct course on entering the denser medium of the water. It is neces sary, however, to observe, that only those rays which enter another meanum
D D obliquely, suffer refraction; for rays which fall perpendicularly are equally attracted on all sides, and, therefore, have no tendency to deviate in any direction. If a shilling, or any other conspicuous but small object, be placed at the bottom of a basin, and the spectator retire to such a distance that the edge of the vessel just prevents its being seen, and the vessel be then filled with water, the shilling will become perfectly visible, though neither it nor the spectator change their place in the slightest degree. In this experiment the spectator looks at the shilling in an oblique direction, and the rays proceeding from it, by which it is rendered visible after the water has been pouned in, are bent towards his eye on entering the air. The greater the density of any medium, the greater is its refractive power; and of two refracting media, that which is of an oily or inflammable nature, will have a greater refracting power than the other. The incident angle is the angle made by a ray of light and a line drawn perpendicular to the refracting surface, at the point where the ray enters the surface; and the refracted angle is the angle made by the ray in the refracting medium, with the same perpendicular continued.
The sine of the angle is a line which serves to measure the angle, being drawn from a point in one leg, perpendicular to the other. In the subjoined figure A C D is the incident angle, H C E the refracted angle, and BCH the angle of deviation; A F is the sine of the angle of incidence; and H G the sine of the angle of refraction. It may seem extraordinary that light should pass more directly through a dense than through a rare medium; but it has been ascertained that light is subject to attraction; and Sir Isaac Newton discovered and demonstrated that this power is the cause of refraction. The truth of this theory is confirmed by the fact, that the change in the direction of the ray commences, not, as might be supposed, when it comes in contact with the refracting medium, but a little before it reaches the surface; and the incurvation augments in proportion as it approaches the medium.
The term lens is given to any transparent substance, as glass, crystal, water, or diamond, having one or both surfaces curved to collect or disperse the light transmitted by it. The lenses in general use are made of glass, and are usually called magnifying glasses. Glass, however, does not possess a greater share of the magnifying property than other transparent substances. Mankind have availed themselves of the principle of refraction to excellent purpose in the construction of lenses; for, by grinding the glass or other substance thinner at the edges than in the middle, those rays of light which would strike upon it in a straight line, or perpendicularly, if it were plain, strike upon it obliquely, and the refraction they suffer, causes them to converge; on the contrary, by making the glass thinner in the middle than at the sides, the rays are refracted the contrary way, and, therefore, become divergent. The nature of refraction through lenses may, perhaps, be rendered more clear, if we reflect that all curved surfaces are composed of straight lines or points, infinitely short, and inclining to each other like the stones in the arch of a bridge.
When parallel rays fall upon a surface of this sort, it is evident that those only which enter the middle part will go on in a straight direction; those which strike the sides will strike them obliquely, and will, consequently, be made to converge. If the surface be a perfect curve, it is clear that only the ray that strikes the centre of the curve will enter it in a straight direction; all the rest will be more or less refracted, according to the degree of obliquity with which they strike the surface, and the whole of the refracted rays will converge to a point called the focus.
Glasses, or lenses, are usually ground for optical purposes into eight different forms. 1. The lens may be flat on both sides, like the pane of a window. 2. It may be flat on one side, and convex on the other. 3. It may be convex on both sides. 4. It may be flat on one side, and concave on the other. 5. It may be concave on both sides. 6. It may be convex on one side, and concave on the other. 7. It may have one side, which must be convex, ground into little facets, while the other side is plain. 8. It may have considerable length in a triangular form. No. 1 is called a plane glass or lens, as its sides are parallel;