INTERFERENCE OF LIGHT. 805 
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. 
Thus at two successive thicknesses of the plate (p), the incident 
rays falling on it in parallel directions, i 7, are reflected partially from 
the first surface, r 7, and partially from the second, r/ r!.. 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 very 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 
