Chap, xxx.] DOUBLE REFRACTION. 397 



a distance of 10 feet from the ophthalmometer, with, 

 his eye on a level with it. In front of the ophthal- 

 mometer is a rod carrying three small rectangular 

 mirrors by means of which three images are thrown > 

 on the cornea from a candle flame placed on one 

 side of the person being observed, whose eye is 

 screened from all light except that reflected from the 

 mirrors. On looking through the ophthalmometer 

 three images (small specks of light) are seen. The 

 plates are then turned till the images are doubled, 

 when, from the angle through which the plates have 

 been turned, the distance between the three images 

 is ascertained. If the size of object and image be 

 known, and the distance of the object from the reflect- 

 ing surface be also known, the radius of curvature of 



o ' 



the surface may be calculated. 



For the radius of curvature is equal to twice the 

 focal distance of the reflecting surface, and f (the 



focal distance) = p ' where p is the distance from 

 the object, o the size of the object, and i that of the 



image. 



CHAPTER XXX. 



DOUBLE REFRACTION, POLARISATION, AND INTER- 

 FERENCE OF LIGHT. 



Double refraction. If a crystal of Iceland 



t/ 



spar, whose ordinary form is rhombohedral, be placed 

 on a piece of paper, in the centre of which a black 

 spot has been marked, on looking down on the crystal 

 two black dots will be seen ; the image of the black 

 dot will be double. If now the crystal be rotated 

 on the piece of paper, one dark spot will be seen to 



