VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. 7ig 



pencil every way like the incident pencil so, excepting that the light will have 

 been somewhat weakened in its passage through the media. 



Other things remaining, let the thickness of the 2d medium be cp, less than 

 cb or CB, the surface parallel to cd being pe ; and the emergent rays w<r will be 

 indeed parallel to the incident as formerly, but the spectrum will fall below the 

 place of the screen where so or os would fall. It will likewise be coloured, as 

 the rays were not yet united at the point o. If the thickness be greater than c^, 

 the spectrum will fall above the line soo^, and the violet and red, after their in- 

 tersection in 0, will have changed sides. 



Other things remaining, suppose the refractive power of the medium ac to be 

 increased, making the extreme rays to intersect before they reach the surface ab ; 

 in that case, let the medium be turned round an axis perpendicular to the plane 

 of refraction (represented by the plane of the figure) in the order of the letters 

 a, b, c, so that the angle of incidence of the rays w, r/-, the line vr, and the 

 angle vor^ may be continually decreasing, till the intersection o falls into the side 

 ab'. and the rays will emerge colourless and parallel to the incident pencil so; 

 above, or below, or in the line soo^, according to the assumed place of the axis 

 of revolution. 



If, on the contrary, the refractive power of the medium ac be diminished, 

 and, with it, the angle of convergence of the extreme rays; the point where 

 they would intersect falling beyond the surface ab-, the medium must then re- 

 volve the contrary way, in the order c, b, a, to bring the point of intersection to 

 the surface ab. But if the refractive power be so small that even when cd be- 

 comes almost coincident with yv, the point of intersection falls still beyond ba, 

 in that case the rays cannot be made to emerge colourless, otherwise than by in- 

 creasing the depth of the medium till its surface passes through the point of in- 

 tersection. And in like manner, when the refractive power of the second me- 

 dium ac is greater than that of ac, making the rays to meet within the medium, 

 as at ^ a point in the line/?e; we may, instead of turning the medium round an 

 axis, cut off the part jba, leaving the surface /je parallel to cd\ and the emergent 

 light will be colourless. 



From these few principles we may determine the phenomena of light trans- 

 mitted through parallelopipids that are contiguous to the air, their position and 

 refractive powers being given. Or we may dispose them so that the emergent 

 light shall, or shall not, be tinged with colours. And we already see (what shall 

 be more distinctly explained below) that if light be transmitted through whatever 

 number of media (a, b, c, &c.) all the refractions may be corrected by the equal 

 and contrary refractions of the same number of the same media (c, b, a,) similar 

 and similarly situated to the former ; provided there be a medium z interposed 



