. Biographies of distinguished scientific men. Scientists. INTERFERENCE OF LIGHT. 305 fain point in space. At this point was placed a sheet of white paper. Each ray, taken by itself, made the paper it wculd be reflected internally at its second surface, and emerge coinciding in direction with the first, but retarded behind it from the thickness traversed in its undulations either by a whole, or half undulation, or some multiples of these,—thus giving either a point of brightness or one of darkness accordingly; or by some intermediate fraction, giving an intermediate shade. And this would go on
. Biographies of distinguished scientific men. Scientists. INTERFERENCE OF LIGHT. 305 fain point in space. At this point was placed a sheet of white paper. Each ray, taken by itself, made the paper it wculd be reflected internally at its second surface, and emerge coinciding in direction with the first, but retarded behind it from the thickness traversed in its undulations either by a whole, or half undulation, or some multiples of these,—thus giving either a point of brightness or one of darkness accordingly; or by some intermediate fraction, giving an intermediate shade. And this would go on alter- nately at successively greater thicknesses of the film, giving a suc- cession of such points or Thus at two successive thicknesses of the plate (/>), the incident rays falling on it in parallel directions, i i, are reflected partially from the first surface, r r, and partially from the second, W rf. According to the difi'erence of thickness traversed, these may be in accordance giving a point of brightness as at -f, or in discordance giving a point of darkness as at °. If two rays or sets of waves, instead of being exactly superimposed be supposed to meet inclined at a very acute angle, in a somewhat similar way they would, at a series of points, alternately conspire or clash with each other, thus giving rise to a series of bright and dark points, the assemblage of which will produce bands or stripes on a screen intercepting the rays. Now as to actual experimental cases, it was in the application of this latter theoretical idea that the inven- tion of Dr. Young was peculiarly displayed. The former case was that alone which seems to have occurred to Hooke in reference to the colours of thin plates, and even this was in his mind but a very indefi- nite conception; nor did it seem at first sight readil}' comparable with such cases as the diflfraction fringes, or still less with the internal bands of a shadow observed by Grimaldi. If Hooke had imagined any the
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