POLARISED LIGHT 



349 



CHAPTER XXH 



POLARISED LIGHT 



198. Double Refraction. This can be demonstrated in a 

 simple rhomb of calcite, by employing the ' focussed parallel 

 beam ' (p. 277) from a pin-hole aperture on the flange- 

 nozzle. The spar ought not to be less than four inches long 

 if the separation is to be easily visible, though its other 

 dimensions may be quite small, such as half an inch in the 

 side. A wider separation is shown by placing an aperture in 

 the stage of the optical front, and a double-image prism on the 

 nozzle, focussing the aperture as small discs upon the screen. 



199. Huygens' Experiment. For this a pair of double- 

 image prisms are mounted somewhat as in fig. 196, N fitting 

 into the nozzle of the optical front, in front of the power. 

 The first prism A is mounted in this 



tube, the second prism B in a cell 

 which can be rotated easily. The 

 two prisms must be chosen to 

 match, and a slot or stage s should 

 be provided between them. Focus- 

 sing the aperture, the alternate dis- 

 appearance and augmentation of 



the images will be seen on rotating the front prism. The slit 

 8 is for the insertion of a film of selenite between the two 

 prisms, which will give tl:e beautiful phenomena of comple- 

 mentary colours. Inserting rather a larger aperture in the 

 stage, so that the pair of discs somewhat overlap, it will be 

 shown that the two overlapping colours always produce white. 

 It is well to have a small slide in which are mounted two 

 such selenites, one giving a green and red, and the other blue 

 &nd yellow. 



FlG. 196. Huygens' Apparatus 



