ON LIGHT AND COLOURS. 



153 



sides onty being affected, the other two at right angles to 

 these are not at all affected by the flexion which has disposed 

 and polarized the two former. Consequently, although an 

 edge placed parallel to the disposing edge and opposite to it 

 acts powerfully on the disposed light, yet an edge placed at 

 right angles to the former edge or across the rays, does not 

 affect them any more than it would rays which had not been 

 subjected to the previous action of a first edge. Thus (fig. 19) 



Rg.19. 



if a b c d be the section of the ray, an edge parallel to a b, after 

 the ray has been disposed, will affect the ray greatly, pro- 

 vided it had been disposed by an edge also parallel to a b. 

 The sides a b and c d, however, are alone affected ; and there- 

 fore the second edge, if placed parallel to a d or b c, will not at 

 all bend the ray more or make images (or fringes) more 

 powerfully than it would do if no previous flexion and dis- 

 position had taken place. Let us see how this is in fact : 

 efgh is the distended disc after flexion, by passing through 

 the aperture of the two-edged instrument (Plate XII.). It is 

 slightly tinged with red at the two ends fg and e h, beyond 

 which, and in the shadow of the edges, are the usual fringes 

 or coloured images by flexion and disposition, c, c, the edges 

 being parallel to eh,fg. Place another edge at some distance 

 from the two, as 3 or 4 inches, and parallel to these two, but 

 in the light, and you will see in the disc a succession of nar- 

 row fringes, parallel to the edges, and in front of the third 

 edge's shadow. These fringes are on the white disc, and 

 their colours are very bright, much more so than the colours 

 of those fringes described in Proposition I., and which are 

 fringes made by deflexion without any disposition. But 

 whether this superior brightness is owing to the glare of the 



