THE ACTION OF ANISOTKOPIC CRYSTALLOID BODIES. 



Elmer, 1 which in other respects is constructed on the same prin- 

 ciple as ours. 



Valentin's object-stage with double rotation (Fig. 177) does not 

 answer satisfactorily the purposes required, as it affords no angular 

 determinations ; it may nevertheless be used in many cases. It 

 is arranged to be clamped to the ordinary stage, and is provided 

 with adjusting screws for centering. The disk liikl can be revolved 

 in its own plane, and likewise round the horizontal axis g. 



II. 



THE ACTION OF ANISOTKOPIC CEYSTALLOID BODIES, 

 VIEWED SEPAEATELY. 



WE will now investigate the phenomena which anisotropic 

 crystalloid bodies produce in polarised light, assuming that the 

 reader is acquainted with the fundamental principles of double- 

 refraction and polarisation as explained in the ordinary hand-books 

 of Physics. We have considered it advisable firstly to develope 

 the relations and laws, which are of special significance in the 

 application of microscopical methods of observation, for crystalline 

 media, in order to prepare the student for the more difficult 

 investigation of organized substances. Many conceptions and 

 expressions, which \ve should otherwise have to explain and 

 determine later on, may be more easily and more directly deduced 

 from the phenomena produced in crystals than would be the case 

 in the more complicated structures of animal and vegetable tissue. 

 They form, at the same time, the natural starting-point for the 

 solution of our task. 



1. THE ELLIPSOID OF ELASTICITY. 



It is well known that the optical action of double-refracting 

 crystals finds its explanation in the property which they possess of 



1 Cf. Ebner: " Untersuchimgen iiber das Yerhalten des Knochengewebes 

 im polarisirten Lichte." (" Sitzungsberichte d. k. k. Akademie d. Wissen- 

 schaften in Wien," third division, 1874.) 



