254 
LORD brougham’s EXPERIMENTS AND OBSERVATIONS 
It may further be observed, that in the fringes or images by disposition and pola- 
rization, the dark intervals disappear at short distances from the point of flexion, 
and that the fringes run into one another, so that we find the red mixed with the 
blue and violet. This is one reason why I often experimented with the prismatic rays. 
3. It follows from the property of light, which I have termed disposition, on one 
side the ray, and polarization on the opposite side, superinduced by flexion, that 
those two sides only 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. Con- 
sequently, 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) \i ah c dhe the 
section of the ray, an edge parallel to a h, after the ray has been disposed, will affect 
the ray greatly, provided it had been disposed by an edge also parallel to a h. The 
sides ah and cd, however, are alone affected; and therefore the second edge, if placed 
parallel to a d or h 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 disposition had taken place. Let 
us see how this is in fact : e f g h is the distended disc after flexion, by passing through 
the aperture of the two-edged instrument (Plate XL). It is slightly tinged with red at 
the two ends fg and eh, beyond which, and in the shadow of the edges, are the usual 
fringes or coloured images by flexion and disposition, e,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 
narrow 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 disc’s light being diminished by the flexion of the first two edges, or 
not, for the present I stop not to inquire. This is certain, that if the third edge be 
placed across the beam, and at right angles to the two first edges, you no longer 
have the small fringes. They are not formed in the direction hg, parallel to the edges 
as now placed. If the double edges are changed, and are placed in the direction h’ g', 
you again have the bright fringes ; but then, if the third edge is now placed parallel 
to h' e\ you cease to have them. Care must, however, be taken in this experiment not 
to mistake for these bright fringes the ordinary deflexion fringes made by one flexion 
without disposition, as described in Proposition I. For these may be perceived, and 
even somewhat more distinctly in the disc than in the full light of the white pencil 
or beam. 
Now are these bright fringes only the flexion fringes, that is fringes by simple 
flexion without disposition? To ascertain this I made these experiments. 
Exp. 1. If they are the common fringes, and only enlarged by the greater diver- 
gence of the rays after flexion, and more bright by the dimness of the distended disc, 
