Reflexion of IAgld from Iceland Spar. 



24-5 



The instruments were firmly attached to a laboratory table, and 

 before commencing the moveable parts were examined and tightly 

 screwed up. Round the base of the table and the foot of the analyser 

 a small quantity of melted paraffin was poured so as to form con- 

 necting links from one part of the apparatus to another. From time 

 to time these links were examined and found to be unbroken. 



To attach the crystal to the table, hard electrical cement was used. 

 The plan finally adopted was to place the crystal on the table, and to 

 fix it by pouring a small quantity of melted wax down the back of it. 



Since the crystal was removed from the table to polish its surface, 

 some means of restoring it to its original position were needed. The 

 following optical method was employed. 



A circular diaphragm with a central pinhole was fitted to the brass 

 tube containing the polarising Nieol. If the pinhole were slightly 

 eccentric, the position of the hole would change as the disk rotated in 

 its plane. To obviate any such alteration, a radius was drawn on the 

 diaphragm which was set so that it was horizontal and always pointed 

 in the same direction. In front of the source of light was placed a 

 screen having a small hole at the same height as the pinhole in the 

 diaphragm. The position of the screen was defined by lines drawn on 

 the table. 



Thus only a single ray of light was allowed to fall on the surface of 

 the crystal, viz., that passing through the apertures in the screen and 

 diaphragm. As these apertures could always be replaced in the same 

 position, the direction of this incident ray was a fixed horizontal 

 straight line. In a similar manner a circular diaphragm w^ith a 

 central pinhole was fitted to the brass tube containing the analysing 

 Nicol, and some distance in front of this tube was placed a screen 

 with a pinhole at the same height as the pinhole in the diaphragm. 

 The position of the screen was defined as before by lines drawn on the 

 table. A radius was drawn on the diaphragm, and also, since the tube 

 itself was moveable, a mark was made on it so that it could be turned 

 into the same position. 



The crystal was placed on the brass table so that its plane was 

 vertical and passed through the centre of the circular top. 



On placing the eye opposite the aperture in the screen facing the 

 elliptic analyser, it was found that a bright dot of light was visible. 

 Thus the horizontal incident ray already mentioned must have been 

 reflected by the crystal surface so that it passed through the apertures 

 in the screen and the diaphragm fitted to the analysing tube. These 

 apertures could be replaced in the same position, and therefore the 

 direction of the reflected ray was a fixed horizontal liue. Consequently 

 the normal to the surface of the crystal bisecting the angle between 

 the incident and reflected rays was a fixed direction. 



Supposing the crystal to have been taken down for polishing, it 



