134 



EXPERIMENTS AND INVESTIGATIONS 



both A and B, which the experiment 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 acted -on the side of the ray 

 on which the second edge acted, I plainly saw ; but I fre- 

 quently changed my opinion whether or not it acted on the 

 opposite side, that is, on the same side with the first edge. 

 K"or 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 experimentum cnicis 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 



> 



\ 



Fig. X. 



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 Gr could be approached as near as possible to each 

 other, and so could F be approached to the plate E on the 

 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 neigh- 

 bouring 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 



