VOL. XXXV.] I'HILOSOPHICAL TRANSACT-IONS. 207 



being changed to a parallelepiped, were covered with a dark paper that had only 

 a small hole in it. 



But to make this more evident, especially to such as are not well acquainted 

 with Sir Isaac's optics, the Doctor explains the manner of the bending of the 

 rays, where they are refracted or reflected. 



Of the Bending of the Rays in their Refraction. — Let dd, fig. 12, represent 

 a dense medium, as glass, whose surface is gg ; and aa a rare medium, as air. 

 Now let us suppose a power to extend all over the surface gg, acting from aa 

 towards dd, in lines penpendicular to the surface gg, very strong in contact, 

 but insensible at a very small distance from the said surfiice, which may be called 

 the attraction of the surface gg, without considering whether it be any real 

 virtue in the said surface, or the action of a medium impelling towards it. Let 

 lines 11, 21, 33, such as express the lines in which the attraction exerts itself, 

 and the line mm, extremely near to gg, the limits of the attraction, beyond 

 which it cannot affect a ray of light. Let the ray of light Ra, moving from a 

 rare medium into a dense in the direction Rr, come towards the surface gg in 

 such an angle, that it may be refracted. When the ray comes to a, by the at- 

 traction at a it will be acted on in the line ab, and by the known laws of me- 

 chanics be turned out of the way into the direction aa, instead of ar : when it 

 is got to b, being acted on in the direction b 4, its new direction will become 

 bb: at c, by the power acting in the line c 5, it will change its direction to cc ; 

 and lastly, at d it will go into the glass in the line dd, continuing in that 

 straight line while it moves in that medium. 



Now if the lines 1 1, 12, 33, n, c, b, a, be infinitely near, as they must be 

 supposed to be, the ray, instead of being broken into the several straight lines 

 ab, be, and cd, will be bent into the curve abed; and the emergent ray dd will 

 make the same angle with the incident ray Rr, as if the refraction had been 

 made at once at the point n, which point may be considered as in the surface 

 GG, because mm has been supposed extremely near that surface : then also may 

 refractions be considered in gross, and rays traced, in all optical propositions, 

 as if there were no such curve as what we have been describing. 



Again, let d, fig. 13, represent the dense medium or glass, and a the rare 

 medium or air ; Ra a ray of light coming out of the dense medium into the 

 rare, in the direction Rr, in which it may be refracted, as for example, in an 

 angle of 30 degrees with the perpendicular pa. Let mm be the line which limits 

 the attraction of the surface gg, which attraction is exerted in lines tending 

 perpendicularly from mm to gg. As soon as the ray of light has emerged at a, 

 it is attracted in the direction ap, and therefore diverted from the line ar, into 

 the new direction aa; at b, it is turned into the line bb; at c, into the line cc; 



VOL. VII. Q Q 



