242 
LORD FROUGHAM’S EXPERIMENTS AND OBSERVATIONS 
that on the polarized side they are not inflected or deflected at all. Their disposition 
on the opposite side is a matter of degree ; their polarization is absolute and their 
flexion null. 
Proposition III. 
The rays disposed on one side by the first flexion are polarized on that side by the 
second flexion, and the rays polarized on the other side by the first flexion are de- 
polarized and disposed on that side by the second flexion. 
This proposition is proved by carefully applying the first experiment of Prop. II. ; 
but great care is required in this experiment, because when three edges are used con- 
secutively, the third edge often appears to act on rays previously acted on by both 
the other two, when it is only acting on those previously acted on by one or other 
of those two. Thus when edge A has inflected and edge B afterwards deflects the 
rays disposed by A, a third edge C may, when applied on the side opposite to B, 
seem to increase the flexion, and yet on removing A altogether we may find the same 
effect continue, which proves that the only action exercised had been by B and C, 
and that C had not acted on rays previously bent by both A and B, which the expe- 
riment of course requires to prove the proposition. I was for a long while kept in 
great uncertainty by this circumstance, whether the third edge ever acted at all. That 
it never aeted on the side of the ray on which the second edge acted, I plainly saw ; 
but I frequently changed my opinion whether or not it acted on the opposite side, 
that is, on the same side with the first edge. Nor could I confidently determine this 
important point until I had the benefit of an instrument which I contrived for the 
purpose, and which, executed by M. Soleil, enabled me satisfactorily to perform the 
eiperimentum crucis as follows : — 
In fig. X. A B is a beam, on a groove (of which the sides are graduated) three 
uprights are placed, the one, B, fixed, the other two, C and D, moving in the groove 
of A B. On each of the uprights is a broad sharp-edged plate, moving up and down 
the upright by a rack and pinion, so that both the plates F, G could be approached as 
near as possible to each other, and so could F be approached to the plate E on the 
