:252 Prof. H. F. Dawes on Image 



the vibrations are uniformly transversal about the axis as 

 centre, and therefore the image formed by this pencil will 

 also seem of constant brightness as the analyser is rotated. 

 If, however, there is any departure from the symmetry of 

 the position of the eye, the brightness of the images will 

 change as the analyser is rotated, so that the images may be 

 •distinguished. Even in this case there is no position of the 

 •analyser which will produce complete extinction of either 

 image. 



Image Formation under other Types of 

 Circumstances. 



1. Image of a point in air by light refracted into a uniaxial 

 crystal through a plane interface normal to the optic axis. — 

 The position of the image in this case may be determined by 

 •a process similar to the above. Thus, in fig. 2, with the 



Fto. & 



R*^ 



source at P, the curvature of the wave P'M before refraction 

 is 1/p ; after refraction it will be a/Vp. But after refraction 

 the wave will proceed as part of a spheroid B/M whose 

 principal curvature has this value, the rays radiating from 

 the centre R of the spheroid. Since the required image is 

 at the point from which the light thus radiates, its position 

 is this centre and its distance from the refracting surface is 

 the corresponding semi-axis of the spheroid. In accordance 

 with (3) this distance is 7"*""^"!^ 

 " a 2 Yp 



c*a 



,(6), 



or, in terms of the indices, 



h 2 p/ni> 



This distance also is 



