OPTICS. 



31 



In flint glass and Water, for example, 

 the ratio of the dispersion for the rays 

 B, C, or for the space B C, which they 

 bound, is as 1 to 2.562. This is found 

 by dividing the differences of the indices 

 of refraction for B and C, in flint glass, 

 by the differences of the same indices 

 for water, as given in the preceding 

 table. 



CHAPTER XII. Inflexion orDiffraction 

 of Light Law of Interference 

 Length* of a U'ave of Light of diffe- 

 rent colours Practical consequences. 



ALTHOUGH the subject of the inflexion 

 and the interference of the rays of light 

 is of a somewhat abstruse nature ; yet 

 it will not be difficult to convey to the 

 reader some distinct, though general no- 

 tions of these curious properties of light. 



If we make a hole in a window shut- 

 ter 1-4 Oth part of an inch in diameter, 

 or, what is better, if we fix in the win- 

 dow shutter of a dark room a small 

 convex lens, of a short focus, we shall 

 obtain a beam of divergent light. If we 

 place bodies of any ki?id in this light, 

 and attentively examine their shadows, 

 we shall find that, on both sides of the 

 shadow, there are fringes of coloured 

 light, the colours being as follows, 

 reckoning from the shadow : 



First fringe ; violet, indigo, pale blue, 

 green, yellow, red. 



Second fringe ; blue, yellow, red. 



Third fringe ; pale Hue, pale yellow, 

 red. 



The distances of these fringes, as well 

 as their intervals, varied as^the num- 

 bers 1, V4- V, X/T, &c. 



"When homogeneous coloured light 

 was used, the fringes were of the colour 

 of the light in which they were held, 

 and their intervals black. Those formed 

 in red light were the largest; those 

 formed in violet the least, and those 

 formed in the green of a middle size : 

 the above fringes are called the external 

 fringes. See Newton's Optics, B. III. 

 Part ]. 



If we now examine the shadow of the 

 body which causes those fringes, we 

 shall find, as was first shown by Ma- 

 raldi, that the shadow is divided by 

 parallel fiinges, which vary in number 

 and in breadth according to the dis- 

 tance from the body that the shadow is 

 examined. These fringes are called the 

 internal fringe*. The e.i ternal and the 

 internal fringes are shown in jig. 34, 

 where AB C D is the shadow, and 1, 2, 

 3, the external fringes. 



As the phenomena now described 



Fig. 34. 



must depend on light, bent, somehow or 

 other, into and towards the shadow 

 A B C D, the name inflexion has been 

 employed to distinguish them, from a 

 Latin word signifying a bending. The 

 name diffraction has also been applied 

 to them. 



In studying the phenomena of inflex- 

 ion, Dr. Young found that if an opaque 

 screen was placed either a few inches 

 before, or a few inches behind, one side 

 of the inflecting body, whose shadow is 

 A B C D, so as to intercept all the light 

 on that side by receiving the edge of its 

 shadow B D, then all the fringes in the 

 shadow A B C D instantly disappeared, 

 although the light passed by the other 

 edge of the body corresponding with 

 A C as before. Dr. Young found that 

 this disappearance was not owing to 

 any diminution of the light, and hence 

 he concluded that the fringes in the 

 shadow A B C D were occasioned by the 

 interference of the rays bent into the 

 shadow at one side of the body with the 

 rays bent i?ito the shadow on the other 

 side. Both Dr. Young and M. Fresnel 

 ascribed the external fringes to the in- 

 terference of the direct rays which 

 passed at a little distance from the in- 

 flecting body with rays which they sup- 

 posed might be reflected from the mar- 

 gin of the inflecting body; but M. 

 Fresnel has since proved that this can- 

 not be the case, and he has, therefore, 

 been under the necessity of supposing 

 that the rays which pass at a sensible 

 distance from the inflecting body deviate 

 from their primitive direction, and con- 

 cur also in the production of the ex- 

 ternal fringes. 



M. Arago has made some important 

 discoveries on the effects of transparent 

 screens upon the coloured fringes. He 

 found that such a screen had the same 

 effect as the opaque screen used by Dr. 

 Younsr, and that very thin transparent 

 screens transferred the fringes from the 

 side where they are formed. This result 

 has very important applications, which 

 our limits will not allow us to notice. 



The law of interference deduced from 

 Dr. Young's experiment may be thus 

 explained. Let us suppose two minute 

 pencils of light radiating from two points 

 close to another, to fall upon the same 

 spot of a piece of paper, in which case 

 they may be said to interfere with one 



