Accouht of Theory Double R fraction. 149 
refraction. The other set of undulations, to which the irregular 
refraction is owing, he conceives to be elliptical, or rather sphe- 
roidal, and to be propagated indifferently both through the ethe- 
real matter, and through the solid particles. Huygens considers 
that the regular arrangement of the particles may contribute to 
the formation of the spheroidal waves, as nothing more is re- 
quired for this than that the light be propagated more quickly 
in one direction than another ; and he was convinced that such 
an arrangement actually exists in Iceland spar 
In proceeding to explain his theory, Huygens supposes AB, 
Fig. 1. Plate 1. (lower part of the plate,) to be the surface 
of Iceland spar, exposed to a beam of light ; and as a per- 
pendicular ray incident upon this surface from a distant lu- 
minary, is, by the theory of undulation^ no more than the 
incidence of a parcel of waves parallel to AB, he considers 
the line BC, parallel and equal to AB, to be a portion of the 
wave of light, of which the points R, H, C, meet AB at 
A, K, hj B. Instead of hemispherical waves, as in ordi- 
nary refractions, he supposes the waves to be now hemisphe- 
roids, whose major semi-axes are oblique to the plane AB. 
Hence SVT will represent an individual wave coming from the 
point A, after RC has arrived at AB. Now, as all the other 
points K, A:, B, will propagate waves similar to SVT, in the 
same space of time that the point A did, the common tangent 
NQ of all these semi-ellipses will be the propagation of the wave 
RC in the transparent body, according to the above theory. 
But the tangent NQ, which is equal and parallel to ABj is 
not directly opposite to AB, but comprehended between AN 
and BQ, conjugate diameters to those which are in the line AB. 
In this way,” says Huygens, I have been able to conceive 
what appeared very difficult, how a perpendicular ray could suf- 
fer refraction in a transparent body,” the wave RC, instead of 
going straight on when it entered the surface AC, extending it- 
self between the parallels AB and NQ.. 
* Huygens considered the existence of two different emanations as rendered 
more probable by the phenomena of rock-crystal, which he found to possess double 
refraction ; but he has maintained, that both the refractions are of the usual kind^ 
and explicable by two spherical undulations, one .of which is propagated a little 
slower than the other. Hence it follows, that he regarded his theory as appU^ 
cable only to calcareous spar. 
