

OK LIGHT AND COLOURS. 125 



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 con- 

 sidered, 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, b' 

 are to be examined, and holding it inclined laterally when 

 the fringes parallel to a, a' are to be examined. It is also 

 convenient to let the white light beyond the fringes pass 

 through ; and for this purpose, a", b" being the figure of the 

 instrument (fig. 2), and the light received on 

 the chart, a hole may be made in its centre 

 op q, 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 op parallel to b" and 

 p q parallel to a". The reddish is farthest from the shadow, 

 the bluish nearest that shadow ; also the fringe nearest the 

 shadow is the broadest, the rest decrease 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 farthest from the direct 

 rays, the violet nearest those rays, the inter- 

 mediate at intermediate distances. This is 





plainly shown in the following experiment. Fi>3. 



Exp. 2. In fig. 3, C represents the image 

 of the aperture when the rays of the prismatic spectrum are 



