Chap. XXX.] POLARISCOPE IN PHYSIOLOGY. 409 



lie parallel to the plane of polarisation of one or other 

 of the prisms ; the rest, which cut those planes at 

 various angles between and 90, appear of a grey 

 colour upon a black ground, the most distinct being 

 those which cut them at an angle of 458. In those 

 parts where the muscular fibres running parallel with 

 one another are arranged in several layers, the colour 

 assumes a whitish tint, passing into yellow. The tint 

 varies with the thickness of the layers, precisely as 

 the succession of colours in Newton's rings, from the 

 centre towards the circumference. If one of the 

 Nicol's prisms be turned to the extent of 90, so that 

 the field becomes clear and attains its maximum 

 brightness, the complementary tints make their ap- 

 pearance. These phenomena, with others . . . ., are 

 equally apparent when the muscular fibres are 

 thoroughly impregnated with, and surrounded by, 

 strongly refracting fluids, as glycerine, turpentine, 

 and Canada balsam. This is essentially owing to the 

 circumstance that the muscle substance is doubly 

 refractile, two systems of undulations propagating 

 themselves according to different laws, and interfering 



<J J !^ 



with one another. 



"This explanation had already been given in 1839 

 by Professor C. Boeck, of Christiania, who was the 

 first that applied the polarising microscope to the 

 investigation of animal and vegetable tissues ; and no 

 other intelligible explanation of the phenomena ob- 

 served has, since that period, been advanced. 



" The next question to determine is, whether the 

 entire substance of the muscular fibres possesses an 

 equal power of double refraction, or whether it is 

 possible to distinguish doubly refracting from isotropal 

 parts. If sufficiently high magnifying powers are 

 employed, and the observations be made on animals 

 which have large sarcous elements, amongst which 

 our large water beetle, the Hydrophilus piceus, is the 



