[ *75 ] 
6. Other things remaining, fuppofe the refractive 
power of the medium ac to be increafed, making 
the extreme rays to interfeCt before they reach the 
furface ab ; in that cafe, let the medium be turned 
round upon an axis perpendicular to the plane of re- 
fraction (reprefented by the plane of the figure) in 
the order of the letters a> b , c , fo that the angle of 
incidence of the rays V v, Rr, the line vr, and the 
angle v or may be continually decreafing till the in- 
terleClion o falls into the fide ab ; and the rays will 
emerge colourlefs and parallel to the incident pencil 
SO ; above, or below, or in the line SO or, accord- 
ing to the afiumed 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 interfeCt falling beyond the furface ab-, 
the medium muft then revolve the contrary way, in 
the order c, b, a ; to bring the point of interfeCtion 
to the furface ab. But if the refraCtive power be fo 
finall that even when cd becomes almoft coincident 
with V v, the point of interfeCtion falls ftill beyond 
ba , in that cafe the rays cannot be made to emerge 
colourlefs, otherwife than by encreafing the depth of 
the medium till its furface pafifes through the point of 
interfeCtion. And in like manner, when the refrac- 
tive power of the fecond medium ac is greater than 
that of A C, making the rays to meet within the me- 
dium, as at q a point in the lin ope> we may, in- 
ftead of turning the medium round on an axis, cut 
off the part pa t leaving the furface pe parallel to cd-, 
and the emergent light will be colourlefs. 
From 
