UPON THE PROPERTIES OF LIGHT. 
237 
between the opposite edges, a, a' are (Plate X. fig. 1 ) the vertical, b, V the horizontal 
edges, s,s' are the screws ; and these may be fitted with micrometers, so as to measure 
very minute distances of the edges by graduated scales B B', B' C. For the purpose of 
the present proposition the aperture only needs be considered, of about a quarter of an 
inch square. The light passing through this aperture is received on a chart placed 
first one foot, and then several feet from the instrument. The fringes are increased 
in breadth by inclining the chart till it is horizontal, or nearly so, when the fringes 
parallel to b, V are to be examined, and holding it inclined laterally when the fringes 
parallel to a, d are to be examined. It is also convenient to let the white light be- 
yond the fringes pass through ; and for this purpose, a", W being the figure of the in- 
strument (fig. 2), and the light received on the chart, a hole maybe made in its centre 
through which the greater portion of the white light may be suffered to pass. 
The fringes are plainly seen to run parallel to the edges forming them ; as ojo parallel 
to W and p q parallel to d'. The reddish is furthest from the shadow, the bluish 
nearest that shadow ; also the fringe nearest the shadow is the broadest, the rest de- 
crease as they recede from the shadow into the white light of the disc. Sometimes 
it is convenient to receive the fringes on a ground glass plate, and to place the eye 
behind it. They are thus rendered more perceptible. 
When the edges are placed in homogeneous light, they are all of the colour which 
passes by any edge ; and two diversities are here to be noted carefully. First, the 
fringes made by the red light are broader than those made by any of the other rays, 
and the violet are the narrowest, the intermediate fringes being of intermediate 
breadths. Second, the fringes made by the red are furthest from the direct rays, the 
violet nearest those rays, the intermediate at intermediate distances. This is plainly 
shown in the following experiment. 
Exp. 2. In fig. 3, C represents the image of the aperture when the rays of the 
prismatic spectrum are made to pass through it. But instead of making the fringes 
by a single edge deflecting, and so casting them in the spectrum, I approach the 
opposite edges, so that both acting together on the light, the fringes are seen in the 
shadow and surrounding the spectrum. These fringes are no longer parallel to the 
shadows of the edges as they were in the white light, but incline towards the most 
refrangible and least flexible rays, and away from the least refrangible and most 
flexible. Thus the red part r of the fringes is nearest the shadow of the edge a'; the 
orange, o, next ; then yellow, y ; green, g- ; blue, h ; indigo, i ; and violet, v. Moreover, 
the fringe rv is both inclined in this manner, so that its axis is inclined, and also its 
breadth increases gradually from v to r. This is a complete refutation of the notion 
entertained by some that Sir I. Newton’s experiment of measuring the breadths in 
different coloured lights and finding the red broadest, the violet narrowest, explains 
the colours of the fringes made in white light as if these were only owing to the dif- 
ferent breadths of the fringes formed by the different rays. The present experiment 
clearly proves, that not only the fringes are broadest in the least refrangible rays, but 
