410 PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



best, it will be immediately seen that only the sarcous 

 elements are doubly refracting, and that the inter- 

 vening material which separates them from one 

 another is isotropal ; for it remains dark in the dark 

 field of the crossed Nicol's prisms, in whatever 

 azimuth the muscular fibre to which it forms a part 

 may be placed. It is just as dark in those muscular 

 fibres which form an angle of 45 with the polarising 

 planes of the prisms, as in those which make an angle 

 of or of 90 with those planes."* 



Professor Briicke's explanation of the behaviour of 

 muscular fibres in polarised light has been quoted, 

 because he is the greatest living authority on the 

 subject. With the aid of what has been said on 

 polarisation and double refraction, it will be quite 

 easily understood. Polarising apparatus is made to 

 fit the ordinary microscope. It consists of two 

 Nicol's prisms, each mounted in a short tube. One 

 (the polariser) is fitted under the stage so that the 

 light from the mirror passes through it before reach- 

 ing the object on the stage. The other prism (the 

 analyser) is fitted in a tube similar to that of the 

 ordinary eye-piece, and is put in place of the ordinary 

 eye-piece when the polarising apparatus is in use. 

 This form of analyser, however, diminishes the field, 

 which is an objection. Other forms of analysers are 

 made, e.g. Abbe's, consisting of an ordinary Huyghens' 

 eye-piece with a doubly refractive prism between, 

 arranged in such a way that one of the two refracted 

 rays is so strongly refracted as to be intercepted by a 

 diaphragm placed above the eye-glass. 



Rotation of plane of polarisation. We 

 have seen that a plate of Iceland spar interposed 

 between two crossed Nicol's prisms will cause illumi- 

 nation of the field, provided the principal plane of the 



* E. Briicke in Strieker's "Human and Comparative Histology." 



