by Reflection from the Pole of a Magnet. 333 



■J- inch thick, -g- wide, and 7£ long, chosen carefully so as to be 

 quite inactive in the polariscope while unstrained. Suppose 

 one of these slips placed between the mirror and the second 

 Nicol, its surface perpendicular to the reflected ray, and its 

 length parallel to X'; and let the glass be stretched in the 

 direction of its length. Stretched glass acts upon trans- 

 mitted light as a positive uniaxal with its axis parallel to the 

 line of extension. In this case, therefore, the extraordinary 

 component sd is retarded relatively to the ordinary y' ; and 

 the method found in the last article gives us this simple rule : — 



To compensate the effect of a small operation R or L, the 

 incident vibration being initially directed along X, at right 

 angles to the plane of incidence, and the reflected vibration 

 being initially cut off by the second Nicol. Leaving the 

 second Nicol in its initial position, and placing the compensa- 

 ting slip between the mirror and the second Nicol, its plate 

 faces perpendicular to the ray, and its length parallel to OX'; 

 stretch the slip along its length in the case of R, and compress 

 it along its length in the case of L. 



The direction X' will be taken as the standard direction of 

 strain : it is at 45° to the plane of reflection, right hand down. 



17. Third experiment. — All the arrangements are as in the 

 first experiment, the angle of incidence about 75°, and the 

 extinction in the polariscope perfect. As the experiment is a 

 purely optical one, the circuit is kept open. To ensure unifor- 

 mity of optical conditions, the block C is kept in position as in 

 the diagram of (7), and the light is viewed through the chink 

 as formerly. 



(1) The first Nicol is turned righthandedly through a very 

 small angle, and the light is distinctly restored. The compen- 

 sating slip is introduced between the mirror and the second 

 Nicol in the manner which has just been fully described. 

 When the slip is stretched along its length, say between 

 closely gripping finger and thumb at each end, with a force 

 which increases continuously from zero up to a certain small 

 value, the light restored by displacement of the first Nicol 

 fades away to pure extinction, reappearing and brightening as 

 the tension increases. When the slip is submitted to a longi- 

 tudinal compression which increases continuously from zero, 

 the light increases continuously and very distinctly from be- 

 ginning to end of the increase of compression. 



(2) The first Nicol is now turned to the left, through the 

 position of extinction, and the light distinctly restored ; and 

 the compensator, kept always in the standard position, is 

 stretched and compressed as formerly. Things are precisely 

 as in the first case, except that the effects of tension and com- 



