Magneto-optic Phenomenon. 299 



The polarizer can be moved along a groove at right angles 

 to this, and is inserted on a pillar provided with sights. This 

 apparatus permits the polarizer to be rotated about a vertical 

 or horizontal axis, and to move in a vertical direction. In 

 this way we are able to bring the axis of the polarizer into 

 the same line as the collimator axis, and to put the spec- 

 trometer with its axis perpendicular to this line. The rings 

 containing the mirrors are, on account of their weight, not 

 placed on the spectrometer table, but rest on a bridge 

 above it. The rings can thus be rotated about a vertical axis 

 and displaced in a horizontal direction, by means of two mov- 

 able sledges at right angles to each other. This allows the 

 centre of the mirror to be placed in the axis of the incident 

 ray, and also permits of an exact adjustment of the mirror for 

 all ano-les of incidence. For angles of incidence smaller than 

 24°, the analyser is also separated from the spectrometer and 

 placed on a cross-piece which can be moved relatively to the 

 circular platform, just as in the case of the polarizer. As the 

 rotations to be measured only amount to a few minutes of 

 angle, it is absolutely necessary that the different parts should 

 be accurately adjusted and securely fastened in their proper 

 places. 



8. In order to measure accurately the small rotations, the 

 Nicol prisms are placed in a parallel beam of light "*. The 

 collimator slit is focused by a telescope situated behind the 

 analyser. At each point of the image of this slit we receive 

 in this manner only those rays which pass in the same direc- 

 tion through the Nicol prisms, and which have therefore in 

 these prisms the same 4 plane of polarization. When the prisms 

 are crossed, a dark band is observed which is displaced parallel 

 to itself along the image of the slit on moving the polarizer or 

 analyser. If the axes of the collimator and Nicol prisms arc 

 parallel to each other, the band occupies the centre of the 

 image of the slit when the Nicols are crossed. All the rota- 

 tions of the plane of polarization are referred to this position 

 of the dark band. In this way we have only to adjust the 

 position of a dark band, instead of, as in the older method, 

 estimating when the intensity of the light is a minimum. 

 For convenience of adjustment the centre of the slit has a fine 

 thread stretched across it. The band is sharply defined if the 

 intensity of the light is not too small. Further, the axes of 

 the Nicol prisms must coincide as nearly as possible. The 

 polarizer is a Lippich prism with plane faces which are in- 



* Sissingh, Dissertation, Leyden, 1885 ; also Arch. Necr. xx. p. 178. 

 Lippich, Wien. Ber. lxxxv. p. 268 (1882). 



