APPENDIX 



187 



disc with a vertical slit be placed on the course of the string, the vibrations 

 will be all obliged to take place La a vertical plane, any side to side movement 

 being stopped by the edges of the slit ^ (fig. 69). 



Light can be polarised not only by the action of crystals, but by reflection 

 from a surface at an angle which varies for different substances (glass 



Fig. 69. 



54° 35', water 52° 45', diamond 68°, quartz 57° 32', &c.). It is also found 

 that certain non-crystalline substances, like muscle, cUia, &c., are doubly 

 refracting. 



The Nicol's Prism is the polarizer usually employed in polariscopes ; it 

 consists of a rhombohedron of Iceland spar divided into two by a section 

 through its obtuse angles. The cut surfaces are polished and cemented 

 together in their former position with Canada balsam. By this means the 

 ordinary ray is totally reflected through the Canada balsam ; the extra- 

 ordinary ray passes on and emerges in a direction parallel to the entering 

 ray. In this polarised ray there is nothing to render its pecuUar condition 

 visible to the naked eye ; but if the eye is aided by a second nicol's prism, 

 which is called the analyser, it is possible to detect the fact that it is 

 polarised. 



This may be again illustrated by reference to our model (fig. 70). 



Fig. 70. 



Suppose that the string is made to ^'ibrate, and that the waves travel in 

 the direction of the arrow. From the fixed point c to the disc a, the string 

 is theoretically free to vibrate in any plane ;^ but after passing through the 

 vertical sUt in a, the vibrations must aU be vertical also ; if a second similar 

 disc b be placed further on, the ^-ib^ations wiU also pass on freely to the other 

 extremity of the string rf, if as in the figure (fig. 70) the slit in 6 be also placed 



' Such a model is, of course, imperfect ; it does not, for instance, represent the 

 splitting of the ray into two, and moreover the polarisation takes place on each 

 side of the slit ; whereas, in regard to light, it is only the rays on one side of a 

 polariser, viz. those that have passed through it, which are polarised. 



^ The imperfection of the model has been explained in preceding footnote. 



