UPON THE PROPERTIES OF LIGHT. 
243 
fixed upright B ; while also each of the three plates could be brought as near the 
rays that passed as was required ; and so could each be brought as near the opposite 
edge of the neighbouring plate. It is quite necessary that this instrument should be 
heavy in order to give it solidity; it is equally necessary that the rack and pinion 
movement should be just and also easy ; for the object is to fix the plates at will, so 
that their position in respect of the rays may be easily changed, and when once ad- 
justed may be immovable until the observer desires to change their position. 
The light was passed under the plate E and acted upon by a b, its lower edge. The 
second plate E was then raised on C so as to act on the side of the rays opposite to 
a h, by its upper edge c d. The fringes inflected by ab were thus deflected by c d, in 
virtue of the disposition given to the side next c d. Then the third plate G, on its 
stand D, was moved so that it could be brought to act by its lower edge ej\ which 
was approached to the rays deflected by cc?, and placed on their opposite side. The 
action was observed by examining the fringes on the chart M. Those which had 
been as o, made by the joint action of the two first edges E, F, were seen to move 
upwards to y as the third edge G came near the rays ; and p was both broader 
than 0, and further removed from the direct rays R R'. In order to make quite sure 
that this change in the size and position of o had not been occasioned by the mere 
action of two plates, as E and G or F and G, it was quite neeessary to remove first E, 
by drawing it up the stand B. If the fringe p then vanished, complete proof was 
afforded that E had acted as well as G. Then F was removed, and if p vanished, 
proof was afforded that F acted as well as E and G. A very convenient variation of 
the experiment was also continued and was found satisfactory. When the joint action 
of F and G gave a fringe, as at q, E being removed up the stand B, then E was 
gently moved down that stand, and as it approached the pencil, which was on its 
way to F and G, you plainly perceived the fringe enlarged and removed from q to p. 
These experiments were therefore quite crucial, and demonstrated that all the edges 
had concurred to form the fringe at/?, the first and third inflecting, the second de 
fleeting. 
The same experiments were made on the fringes formed by the deflexion of the 
first edge and the inflexion of the second, and the deflexion of the third. 
It is thus perfectly clear that the rays bent by the first edge and disposed on 
their side opposite to that edge, are bent in the other direction by the second edge 
acting on that opposite side, and are afterwards again bent in the direction of the 
first bending by the action of the third edge upon the side which was opposite the 
second edge and nearest the first edge. But this side is the one polarized by the 
first edge, and therefore that side is depolarized by the action of the second edge. 
Hence it is proved that the rays polarized by one flexion are depolarized by a second; 
and as it is proved by repeated experiments that no body placed on the same side of 
the rays with any of the bending bodies, whether the first or the second or the third, 
exercises any action on those rays, it is thus manifest that any one flexion having 
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