ON LIGHT AND COLOURS. 131 



the same side of the ray with the first edge. These two edges 

 are exactly like two successive particles of the same surface 

 near to which the rays pass. Consequently the two experiments 

 are not similar but identical ; and thus the known fact of the 

 edge and the back of a razor making the same fringes, proves 

 the polarization of the rays on one side. Thus the proposition 

 is proved as to polarization. 



E.\p. 3. The proposition is further demonstrated, as regards 

 disposition, in the clearest manner by observing the effect of 

 two bodies, as edges, whether placed directly opposite to each 

 other while the rays pass between them so near as to be bent, 

 or placed one behind the other but on opposite sides of the 

 rays. Suppose the edges directly opposite one to the other, 

 and suppose there is no disposition of the rays to be more 

 easily bent by the one edge in consequence of the other edge's 

 action. Then the breadth and distension and removal of the 

 fringes caused by the two edges acting jointly, would be in 

 proportion to the sum of the two separate actions. Suppose 

 that one edge deflects and the other inflects, and suppose that 

 inflection and deflexion are equal at equal distances, following 

 the same law ; then the force exerted by- each edge being 

 equal to d, that exerted by both must be equal to 2 d. But 

 instead of this we find it equal to 5 (7, or 6 d, which must be 

 owing to the action of the two introducing a new power, or 

 inducing a new disposition on the rays beyond what the 

 action of one did. 



If, however, we would take the forces more correctly 

 (fig. 10), let A and B be the two edges, and let their spheres 



of flexion be equal, A C ( = a) being A's sphere of inflexion 

 and B's sphere of deflexion ; B C ( = a) being A's sphere of 

 deflexion and B's sphere of inflexion ; and let the flexion in 

 each case be inversely as the mih power of the distance. Let 



K 2 



