BEHAVIOUR OF MUSCULAR FIBRES IN POLARISED LIGHT. 239 



prism to the axis of which it is perpendicular. The sarcous 

 elements are consequently positive like rock crystal. 



The proof of this is obvious. Since the light passes 

 through the first prism at right angles to its principal axis, the 

 plane of vibration of the extraordinary ray is perpendicular to 

 the principal axis, that of the ordinary ray parallel to the 

 principal axis, or at an azimuth of 90 from the former. 

 The extraordinary ray precedes the ordinary, and interference 

 phenomena exhibit differences of shade, which are dependent 

 on the thickness of the prism, and the wave-lengths of the 

 ordinary and extraordinary ray. The two rays emerge from 

 the first prism with this difference of shade, and as they pene- 

 trate into the second, which crosses the first at 90, the ordinary 

 ray can only produce vibrations parallel to the axis, the 

 extraordinary only those which are at right angles to the 

 principal section. Thus the vibrations which constitute 

 the ordinary rays of the first prism form the extraordinary 

 in the second, and vice versa. Since now in the second prism 

 the extraordinary ray is propagated with as much increase of 

 rapidity as in the first, it is clear that the difference of velocity 

 must diminish until equal thicknesses of the two prisms are 

 traversed ; that it is then = ; and if the passage through the 

 second prism is longer than through the first, it increases with 

 opposite signs. 



If now a doubly refracting body be placed on the upper 

 prism, the optic axis of which is parallel with the principal 

 axis of the crystal, the ordinary ray of this upper prism will be 

 propagated as an ordinary ray in it ; and the extraordinary as 

 an extraordinary ray. It acts thus upon the difference of 

 shade as a thickening, if the ordinary in it, as in the prism 

 itself, is propagated less rapidly than the extraordinary; but 

 if the opposite occur, it must operate in the same manner as a 

 thinning of the prism with the principal axis of which its 

 optic axis is parallel. 



An important question still rerAains, which can be solved by 

 the help of the polarising microscope: Are the sarcous ele- 

 ments to be regarded as single and individual elementary 

 bodies, or as groups of solid bodies capable of being variously 

 disposed ? If the muscles contract, the fibres are seen to 



