250 



Prof. H. F. Dawes on Image 



are a, y, y. The radius of curvature of the spheroid in any 

 normal section at its pole is 



r = y 2 /x = A/a 2 = ?2 1 2 ^/?i 2 2 , (3) . 



The diagrams are drawn to correspond to a crystal of 

 principal indices ?i 1 = 4/3 and ?i 2 = 2. The trace of the wave 

 is shown at incidence and just after refraction ; also the 

 circle of curvature of the elliptic wave is shown in certain 

 cases. The traces for actual wave-positions are indicated by 

 continuous lines and for " virtual " wave-positions by dotted 

 lines. 



For the example under consideration the source of light P 

 lies within the crystal at a distance p from the refracting 

 surface at M (fig. 1). The ordinary waves form an image 



Fig. 1. 



at a distance q from M such that p\q is equal to n l9 (4), as 

 in ordinary isotropic substances. The extraordinary waves 

 also form an image, but it is not situated at Q, although the 

 two sets of waves travel with the same velocity along PM. 

 (This is contrary to the statement usually made in discussions 

 of this subject, viz., that only one image is seen on looking 

 along the axis into a crystal. It is true that the eye seems 

 to see only one image, but this is because the one image is 

 vertically beneath the other and the depth of focus of the 

 eye is very great compared with the distance between the 

 images. The presence of the two images may, however, be 



