228 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



of the whole field ; between these positions the position of maximum 

 darkness of the half-prism lies midway. As the adjustment for uniform 

 illumination admits of greater precision than that for maximum dark- 

 ness, this analysing arrangement is more accurate than a single nicol. 

 It is, moreover, especially advantageous to have two positions of adjust- 

 ment, and to take their mean. 



The reflecting plane of the glass body is, by means of an auto- 



Fig. 23. 



collimation ocular, provided with cross-threads, set parallel to the 

 rotation axis of the goniometer, whereby the polarising planes of the 

 analyser and polariser are parallelised with great accuracy, being per- 

 pendicular to the goniometer rotation axis. Consequently, it is likewise 

 possible, with great accuracy, to calibrate the drum divisions of the 

 compensator, and to so adjust the rotatory compensator about the 

 telescope axis that the optical axes of its quartz plates lie parallel, being 

 both perpendicular to the goniometer axis. 



In adjustments with the compensator, the movable quartz wedge is 

 first pushed far enough to make the half-prism show dark ; it is_ then 

 further pushed, first in one, afterwards in the other direction, until the 

 field shows equal illumination. The mean of these two adjustments of 

 the wedge gives the position of dark adjustment more accurately than 

 if one attempted to get it by one position of the nicol. 



