152 



MALUS. 



crystal 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 E in the annexed figure (1.): if the 



Fig. 2. 



Fig. I. 



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 raj r s. 



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 s s, 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 resolution of motions, represented by the 

 lines o, 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 o 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 o and o e> In like 

 manner E is resolved into E parallel to s' s', and E e perpendicular to 

 it. According to the inclination given to s' s', relative to s s, the 



