ON LIGHT AND COLOURS. 



161 



PLATE xrn. 



Is the instrument with four surfaces. A D, a d are two 

 parallel plates, moving horizontally by a rack and pinion E. 

 Each plate has an edge composed of four surfaces ; A, a, a 

 sharp edge or very narrow surface ; B, ft, a flat surface ; C, c, 

 a cylindrical surface of large radius of curvature, and so flat ; 

 D, d, one of small radius, and so very convex : this is re- 

 presented on the figure by A' B' C' D' beside the other. Care 

 is to be taken that A B C D and a b c d be a perfectly straight 

 line, made up of the sharp edge, the plane surface and the 

 tangents to the two cylinders. H is a plate with a sharp and 

 straight edge, op, which can be brought by its handle F to 

 come opposite to the compound edge abed, when it is desired 

 to try the flexion by the latter, without another flexion by an 

 opposite compound edge, but only with a flexion b} 7 a recti- 

 linear simple edge. 



