VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. 2QQ 



moving in the rare, towards the dense medium, in the direction Ra towards 

 r ; if instead of an attraction at the surface of the glass mm, there be supposed 

 a repellent force, whose limits are gg ; then will the ray, by the repulsion of 

 the surface mm, be bent into the curve abcdefghiklm, in the same manner as 

 we showed it would be under the surface gg, when opPG was considered as a 

 dense medium. Hence it follows, that a ray moving in the air, is reflected 

 from a specular surface of glass, or any other mirror, opaque or diaphanous, 

 without touching the said surface. 



N. B. That the same power may, under different circumstances, attract to 

 and repel from the same surface, shall be made out in the remaining part of this 

 paper ; but now taking such a power for granted, we will proceed in consider- 

 ing the flexure of rays of light. 



Let us suppose a prism acb, fig. 1 5, to have the attracting power of its in 

 ferior surface extend as far as the line mm; if another prism gdf, the attracting 

 force of whose upper surface extends as far as nn, be brought very near to the 

 first prism; where the attracting powers of the prisms interfere, they will de- 

 stroy each other, because they act in contrary directions ; and thereby the limits 

 of attraction of each of the surfaces will be contracted ; the power of ab ex- 

 tending no further than nn, and that of df no farther than mm, while the 

 space nnmm loses all the force that it had (and would have on the removal of 

 either prism) to turn a ray of light, moving obliquely, out of its direction. 



Now in this situation of the prisms, a ray of light entering the surface cb at 

 right angles, will go through the 2d prism also at right angles, not exactly in the 

 same line, but in a line parallel to the direction of the incident ray: for example, 

 let the ray Ra, not refracted at, because perpendicular to, the surface cb, emerge 

 from the first prism at a, in the direction ar; its changed direction at a will be- 

 come aa, and at b, bb, or rather the ray will be inflexed in the curve ab ; and 

 at b getting out of the power of the attraction of the surface ab, it will, for 

 the reasons before given, move in a straight line from b to c, where it will be 

 bent again the contrary way in the curve cd, of the same kind as ab, and lastly 

 emerge in the direction dd, parallel to the first direction Rr. From hence it 

 follows, that when the prisms are brought so near as to touch, their mutual at- 

 tractions destroying each other, the rays of light will not be bent, but pass 

 through the two prisms (which in this case perform the office of a parallelo- 

 piped) in the same direction with which they came into the first prism, and con- 

 sequently produce no colours ; contrary to what is afiirmed by Rizzetti (page 78, 

 79> &c.) ; and when the rays Ra fall obliquely on the surface cb, the effect of 

 their refraction at their immersion at s, to produce colours, is taken off by the 

 refraction which they suffer at their emersion at z. 

 Q a 2 



