APPENDIX 



223 



ing bodies are called ariisotropouA ; singly refracting bodies, 

 The effect of polarisation may be very roughly illustrated by a model. 



If a string be stretched as in the figure, and then touched with the finger, 

 it can be made to vibrate, and the vibrations will be free to occur from above 

 down, or from side to side, t>r in any intermediate position. If, however, a 

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

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

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



Fm 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 

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

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

 refracting. 



Nicol's Prism is the polariser usually employed in polariscopes ; it con- 

 sists of a rhornbohedron 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 



Fir,. 70. 



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 peculiar 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). 



Suppose that the string is made to vibrate, and that the waves travel in 

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



1 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. 



