INTERFERENCE OF LIGHT. 305 



tain point in space. At this point was placed a sheet of 

 white paper. Each ray, taken by itself, made the paper 



it would 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 bands. 



X/ 



Thus at two successive thicknesses of the plate (p), the incident 

 rays falling on it in parallel directions, i z, are reflected partially from 

 the first surface, r r, and partially from the second, rt rl. According 

 to the difference of thickness traversed, these may be in accordance 

 giving a point of brightness as at +, 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 veiy indefi- 

 nite conception; nor did it seem at first sight readily comparable with 

 such cases as the diffraction fringes, or still less with the internal 

 bands of a shadow observed by Grimaldi. If Hooke had imagined 

 any theoretical views of this kind, it was probably confined to the 



