680 



The following expedient proved very convenient in the mutual 

 adjustment of the parts of the goniometer. The sledge on which 

 the silver mirror stands, which served for these adjustments, was 

 fastened to a socket, which fits over a conical pivot in the middle 

 of the goniometer circle. This pivot can be levelled with three 

 adjusting screws. The mirror with the socket can in this way easily 

 be placed on the goniometer and can be removed from it ^). 



As polarizer a nicol is used with pretty large oblique end-planes, 

 which only gives a small deviation to the rays of light, which pass 

 through it. This amounts to 2'. 5. 



For the adjustment of the compensator we refer again to the 

 investigation of one of us '). 



The polarisation planes of eacli of the two compensator wedges 

 are placed in the required position, i. e. parallel to the plane of 

 incidence and normal to it '). 



To bring the movable wedge in the required position, we make 

 use of the images of the collimator slit, which are formed in tlie 

 eye-piece behind the goniometei-. There are formed three pairs of 

 images. The images of each pair coincide, if the planes of the 

 wedges are parallel. The principal positions of the nicols, in 

 which their planes of polarisation are parallel to the plane of 

 incidence or normal to it, have been determined both in the \ ertical 

 and in the horizontal position of the goniometer circle. The azimuth 

 of the polarizer is called 0, when the light, that the polarizer trans- 

 mits, vibrates noi-mal to the i)lane of incidence, that of the analyzer, 

 wiien the direction of vibration of the transmitted light is pallallel 

 to the plane of incidence. We obtained successively in the horizontal 

 and vertical position of the goniometer circle : mean 



Polarizer in azimuth 0° 8l°57', 8r43' ; 8r50' 

 Analyzer „ „ 0° 86°33', 86°35' ; 86°34' ^). 



Considering the inevitable errors of observation, the agreement 

 may be called satisfactory. 



1) For fuller details compare J. J. Haak, Thesis for the doctorate, Amsterdam, 

 1918. 



2) R. SissiNGH, loc. cit. 



') It is noteworthy, that it is supposed both in the investigation of R. Hennig, 

 Gölt. Nachr., 13, 365, 1887, as in that of P. Drude, Wien. Ann., 34, 489, 1888; 

 86, 532, 1889; 39, 481, 1890, that the principal sections of the wedges are normal 

 to each other and only the angle between the principal section of one of the wedges 

 and the plane of incidence is determined and taken into account. Gf. Sissingh in 

 Bosscha's Textbook of physics, Light, II, p. 555, note 2. 



^) All the experiments have been made by J. J. Haak. Compare for further 

 details Mr. Haak's thesis for the doctorate, Amsterdam, 1918. 



