ON LIGHT AND COLOURS. 135 



that their position in respect of the rays may be easily 

 changed, and when once adjusted 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 F was then raised on C 

 so as to act on the side of the rays opposite to a b, by its 

 upper edge c d. The fringes inflected by a b 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 ef t which 

 was approached to the rays deflected by erf, 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 p as the third edge G came near the rays ; 

 and p was both broader than o, and further removed from the 

 direct rays EE'. 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 necessary to remove first E, by drawing it up the 

 stand B. If the fringe p then vanished, complete proof was 

 afforded that Ehad acted as well asG. ThenF 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 tried 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 there- 

 fore quite crucial, and demonstrated that all the edges had 

 concurred to form the fringe at p, the first and third in- 

 flecting, the second deflecting. 



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 



