318 



OPTICS. 



same effects would follow as in the impulse of other 

 elastic Ixxlies ; but the angle of incidence could 

 not be equal to the angle of reflection, unless the 

 particles of light were perfectly elastic, or the bodies 

 on which they impinged were perfectly elastic. Now, 

 we know tliat the bodies on which these particles 

 impinge are not perfectly elastic ; and also that, if 

 the particles of light were perfectly elastic, the diffu- 

 sion of light from the reflecting bodies would be very 

 different from its present appearance; for, as no body 

 can be perfectly polished, the particles of light, which 

 are so inconceivably small; would be reflected back 

 by the inequalities on the surface in every direction ; 

 consequently we are led to this conclusion, that the 

 reflecting bodies are possessed of a power which acts 

 at some little distance from their surfaces. If this 

 reasoning is allowed to be just, it necessarily follows 

 that, if a ray of light, instead of impinging on a body, 

 should pass so near to it as to be within the sphere of 

 that power which the body possesses, it must neces- 

 sarily suffer a change in its direction. Actual expe- 

 riments confirm the truth of this position ; and to the 

 change in the direction of a particle of light, owing 

 to its nearness to a body, we give the name of inflec- 

 tion. From one of these experiments made by Sir 

 Isaac Newton, the whole of this subject will be easily 

 understood. At the distance of two or three feet 

 from the window of a darkened room, in which was 

 a hole three-fourths of an inch broad to admit the 

 light, he placed a black sheet of pasteboard, having 

 in the middle a hole about a quarter of an inch 

 square, and behind the hole the blade of a sharp 

 knife, to intercept a small part of the light which 

 would otherwise have passed through the hole. The 

 planes of the pasteboard and -blade were parallel to 

 each other ; and when the pasteboard was removed 

 to such a distance from the window as that all the 

 light coming into the room must pass through the 

 hole in the pasteboard, he received what came 

 through this hole on a piece of paper, two or three 

 feet beyond the knife, and perceived two streams of 

 faint light shooting out both ways from the beam of 

 light into the shadow. As the brightness of the di- 

 rect rays obscured the fai-nter light, by making a hole 

 in his paper, he let them pass through, and had thus 

 an opportunity of attending closely to the two streams, 

 which were nearly equal in length, breadth, and 

 quantity of light. That part which was nearest to 

 the sun's direct light was pretty strong for the space 

 of about a quarter of an inch, decreasing gradually 

 till it became imperceptible ; and, at the edge of the 

 knife, it subtended an angle of about 12, or at most 

 14. Another knife was then placed opposite to the 

 former, and he observed that, when the distance of 

 their edges was about the ith part of an inch, the 

 stream divided in the middle, and left a shadow be- 

 tween the two parts, which was so dark that all light 

 passing between the knives seemed to be bent aside 

 to one knife or the other. As the knives were brought 

 nearer to each other, this shadow grew broader, till, 

 upon the contact of the knives, the whole light dis- 

 appeared. Pursuing his observations upon this ap- 

 pearance, he perceived fringes, as they may be termed, 

 of different coloured light, three made on one side by 

 the edge of one knife, and three on the other side by 

 the edge of the other ; and thence concluded that, 

 as, in refraction, the rays of light are differently 

 acted upon, so are they at a distance from bodies by 

 inflection ; and by many other experiments of the 

 same kind, he supported his position, which is con- 

 firmed by all subsequent experiments. We may na- 

 turally conclude that, from this property of inflection, 

 some curious changes will be produced in the appear- 

 ance of external objects. If we take a piece of wire 

 of a less diameter than the pupil of the eye, and 



place it between tin; eye and a distant object, the 

 latter will appear magnified ; for the rays by which 

 the object would have been otherwise seen are inter- 

 cepted by the wire, and it is now seen by inflected 

 rays, which make a greater angle than the direct 

 rays 



Natural Phenomena. The most interesting of these 

 is the rainbow, which consists of two bows, or arches, 

 extended across the part of the sky, which is opposite 

 to the sun. The innermost of the bows, and which 

 is most commonly seen by itself, it being the princi- 

 pal rainbow, is part of a circle whose diameter is 

 82, and is nothing more than an infinite, number of 

 prismatic spectra of the sun arranged in the circum- 

 ference of a circle, the colours being the very same, 

 and occupying the same space as in the spectrum 

 produced from the sun's light. The red rays form 

 the outermost portion, and the violet rays the inner- 

 most portion of the bow. 



Let A, fig. 23, be a drop of rain, and S d a ray 

 from the sun falling upon or entering it at d, it will 

 not go to c, but be refracted to n, where a part will 

 go out, but a part also will be reflected to y, where it 

 will go out of the drop, which, acting like a prism, 

 separates the ray into its primitive colours, and 

 the violet will be uppermost, the red lowermost. 

 These colours are all at fixed angles ; the least 

 refrangible, or red, makes an angle with the solar 

 incident ray, equal to little more than 42 ; and 

 the violet or most refrangible ray, will make with 

 the solar ray an angle of 40. The ray S d 

 would go to f c, therefore the angle made with the 

 red ray is S/y, and that made with the violet ray is 

 S c g ; the former is 42 2', the latter 40 17'. The 

 upper, or external bow is formed by two refractions 

 and two reflections: suppose the ray T r to be 

 entering the drop B at r. It is refracted at r, re- 

 flected at S, reflected again at t, and refracted as it 

 goes out at u, whence it proceeds, being separated, 

 to the spectator, at g. Here the colours are reversed ; 

 the angle formed by the red ray is 51, and that 

 formed by the violet is 54. 



The external or secondary bow is much fainter 

 than the other, and has the violet outermost and the 

 red innermost. It is part of a circle 104 in dia- 

 meter. As the rainbow is never seen unless when 

 the sun is shining, and when rain is falling between 

 the spectator and the part of the horizon where the 

 bow is seen, it is obvious that it depends upon the 

 decomposition of the white light of the sun, by the 

 refraction of the drops of rain and their subsequent 

 reflection within the drops an explanation suffi- 

 ciently adequate, from the fact that rainbows are 

 produced by the spray of waterfalls, and may be 

 made artificially by scattering water with a brush or 

 syringe when the sun is shining. The primary bow 

 is the effect of one reflection and two refractions of 

 the sun's rays by the drops of rain : the secondary 

 one is formed by two reflections and two refractions. 

 Within the primary rainbow, and immediately in 

 contact with it, there have been seen what are 

 called supernumerary rainbows, each of which con- 

 sists of red and green. Their origin has not been 

 explained. Lunar rainbows have been seen ; but 

 they differ in no respect from those formed by the 

 solar rays, excepting in the faintness of their light. 

 A halo is a circle, either composed of white light, 

 or consisting of the prismatic colours, which is occa- 

 sionally seen round the sun or moon. Parhelia are 

 mock suns, which appear at places where two haloes 

 or arches of luminous circles about the sun intersect 

 each other. The prismatic haloes which are some- 

 times visible about the sun and moon, in fine weather, 

 when white, thin, fleecy clouds are floating in the 

 atmosphere, are called coronce. Owing to the daz- 



