THE APPARATUS FOR ROTATING THE OBJECTS. 



315 



Beneche's No. 2 eye-piece, under the relations represented in Fig. 

 174, the Nicol may also be replaced by a simple prism of spar, by 

 which the image given by the ordinary rays is displaced so far 

 laterally that the eye situated in the axis sees only the image 

 given by the extraordinary refracted rays. A prism of this kind is 

 in many cases preferable to Nicol's, since it renders possible the 

 comparison of the complementary ordinary image with the extra- 

 ordinary one, and facilitates the true interpretation and designation 

 of the polarisation colours. For more convenient observation of 

 the ordinary image the mounting is arranged to admit of a slight 

 lateral motion of the prism, sufficient to enable the observer to view 

 the one or the other image in the direction of the axis of the 

 Microscope. Obviously, too, it must satisfy the demands which are 

 otherwise made of the analyser. 



In Hartnack's analyser of recent construction (Fig. 175) the 

 mounting is united to the eye-piece b c, and there 

 is a graduated disk a, in which the tube with 

 the lenses and the analysing prism can be rotated. 

 The pointer d, which rotates with the prism, 

 indicates the angular magnitude of the revolu- 

 tion. Leitz, and Seibert and Krafft, have also 

 constructed this apparatus of Hartnack's, adding a 

 vernier and crossed wires ; and Merz, Wasserlein, 

 and Verick have adopted the graduated disk. 

 Such contrivances are useful for certain observa- 

 tions (for instance, on circular polarisation) ; but 

 in most cases with crossed Nicol s and stationary 

 plates of selenite, it is even more important to bring the object by 

 means of a rotating plate to the different positions with regard to 

 the planes of polarisation of the Nicols, and thus be enabled to 

 determine the angles. 



FIG. 175. 



3. THE APPARATUS FOR ROTATING THE OBJECTS. 



The observation of the changes which the polarisation colours 

 undergo on rotating the objects is so important in researches with 

 polarised light, that the usual devices for effecting the rotation are 

 not adequate for the purpose. A complete polarising Microscope 

 should be provided with means for securing a slow rotation round 



