6. Rays of light do not consist of contiguous particles or portions.—When rays of light, issuing from different sources, cross one another, they do not seem to disturb each other's motion, ur produce indistinct or imperfect vision, which would undoubtedly be the case if they consisted of contiguous portions. If a number of lumi nous balls were discharged from a cannon with great rapidity, they would appear to occupy an uninterrupted line, though they were actually moving at a considerable distance from one another. This effect, which may be shown more simply by whirling round a burning stick, sn as to make it form in the air a complete circle of light, arises from the duration of the impression of light upon the retina. 1\1. d'Arcy has shown, that the im pression of light continues eight-thirds, or nearly, one eighth of a second ; and as light moves through 26,010 miles during that small portion or time, it is evident that constant vision will be maintained by a succession of luminous particles or luminous waves 26,010 miles distant from each other.
The word Dioptrics, from the Greek through, and 07C7014.41, to sec, is that branch of optics which treats of thc refraction of light, or or the changes of direction which light experiences in passing through transparent bodies.
On the General Principles of Refraction.
Dd.. I. The term 7?efraction, from the Latin Refractio, is used to denotc the change, similar to breaking or bending, which takes place in a ray of light when it falls from air upon any substance, such as Water or Glass, or from these substances into air.
Def. 2. A Medium is the name given to any portion of space or of matter, which either allows light to pass through it, or reflects it from its surface. Those media which transmit light, arc called refracting or transpa rent, oz. diaphanous uzedia ; and those which either re flect the rays, or absorb them completely, so as to trans mit no light, are called opaque media. Every medium except a vacuum is a reflecting medium, unless when they are reduced to such tni.,ute films, that they cease to have the power oE driving back the light which falls upon them.
Def. 3. Light or its rays are said to be Incident upon a Medium, or upon a body, when they fall upon that me dium or body.
When a ray of light falls from a rarer upon a denser -efracting medium, it suffeis such a change in its di rection, that the sine of the angle of incidence is to the sine of the angle of refraction in a constant ratio, and the incident and the refracted ray are in the same plane.
Let AC, PLATE CCCCXXVI1I. Fig. 1. be a ray of light incident on the surface RS of water, or any other medium. This ray is found to suffer such a change in its direction, that instead of proceeding in a line which is a prolongation of AC, it is bent or refracted at C into the direction CE. In like manner, another ray aC, in
cident on the same point C. is found to be bent or refract ed into the line Ce. Through the point C draw the line PCQ perpendicular to the refracting- surface RS, and upon C as a centre describe a circle ABEa, then the an gle ACP is called the ilngle of Incidence ; AC ECQ the 4ngle of Refraction, lor the ray AC; while aCP is the angle of incidence, and eCQ the angle of refraction of the ray aC. If we now compare the angles of refrac tion with their corresponding angles of incidence, we shall find no particular relation between them ; except ing that the one increases with the other, and vice versa ; but if we compare the Sines of these angles, viz. All, the Sine of the angle of incidence ACP with EF, the Sine of the ungle of refraction ECQ, and also a d with e f, we shall find that tile ratio of the one to the other is constant, whatever be the value of tlle angles of incidence or refraci ion.
If the surface RS is that of Water, All will be to EF, or a d to e f, as 4 to 3 nearly.
If the surlace RS is that of Crown glass, AD will be to EF, ard a d to c f, as 3 to 2.
If the surface RS is that of Zircon or Suip/2ur, AD will be to EF, and a d to e f, as 2, to 1, or I to ,12.
If the surface RS is that of Diamond, AD will he to EF, and a d to e f, as 1 to Hence it folloNvs, that the Sine of the angle of Incidence is to the Sine of the angle of Refraction in a constant ratio.
By admitting the light through a small apetture at A, zo as to pass through another aperture at C, and fall upon the bottom of thc vessel at E, it will be found that the three points A, C, E, are always in the same straight line, whatever be the angle of incidence ACP. Hence -it follows, that the refracted ray CE, and the incident ray AC, are always in the same plane.
Cox. When a ray passes from a rarer into a denser medium, the refraction is always made towards the per pendicular. This follows from the angle of refraction always being less than the angle of incidence.
Supposing the sine of the angle of refraction to be al ways 1, then the sine of the angle of incidence will be nearly 1.33 in water, and 1.50 nearly in glass. In this case the sine of the angle of incidence, or 1.33 and 1.50, is called the .Index of Refraction, or the Co-efficient of Refraction ; and this mode of expression is universally adopted to indicate the relation between the sines of in cidence and refraction. When \VC are told, for example, that the refractive power, or the Index of refraction be longing to diamond, is about 2.5, we learn, that if 2.5 is to represent the sine of the angle of incidence, the sine of the angle of refraction Nvill be 1.