vio 
152 MALUS. 
erystal of Iceland spar, has the properties of an ordinary 
ray if we only make it turn round itself through a quar- 
ter of a revolution. 
within the crystal to be of an elongated form; it may be represented 
by a short straight line, as o and £ in the annexed figure (1.): if the 
Fig. 2. 
Fig. 1. 
“¢ 
NS 
0 E 
\ 
Fae 
rhomb represent a section of the crystal looking down perpendicu- 
larly upon it, and supposing the light to fall on it in the same perpen- 
dicular line, s s will be the projection of its principal section, and the 
short lines o and E will be the projections of the sections of the ordi- 
nary and extraordinary rays. 
Now let us conceive this first crystal to retain its position, and its 
principal section s s to remain parallel to itself, as in jig. 2, and a 
second crystal placed upon it, having its principal section s/ s/ in- 
clined at any angle to the former; then supposing the sections o and 
E to remain as before, relatively to ss, that is one parallel, and the 
other perpendicular to it, when those rays enter the second crystal, 
the effect is that they can only pass through it in such portions as are 
either parallel or perpendicular to its principal section s/ s/._ It be- 
comes then simply a case of resolutivn of motions, represented by the 
lines 0, E, and it seems nearly impossible to imagine this without 
associating it with vibrations. At all events, the only way of con- 
ceiving the matter is to admit that in some way 0 is simply resolved 
into two components at right angles; one in the plane s s/, the other 
perpendicular to it, which are represented by o, and o,, In like 
manner E is resolved into H, parallel to s/ s!, and &, perpendicular to 
it. According to the inclination given to s/ s/, relative to s s, the 
s’ 
. 
