MINUTE STRUCTURE OF STRIATED MUSCLE-FIBRES. 15 



brought to light other differences besides that of the 

 mere alternation of lighter and darker streaks. Of 

 the highest importance as explaining the structure of 

 muscle-fibres are the researches of E. von Briicke into 

 the phenomena exhibited by muscle-fibres in polarised 

 light. According to modern physical views, light de- 

 pends on the vibrations of ether, an impalpable matter 

 spread throughout the universe and present in all bo- 

 dies. These vibrations always proceed at right angles 

 to the direction in which motion is propagated. With- 

 in this imaginary plane at right angles to the ray of 

 light, an ether particle may vibrate in the most diverse 

 directions. Under certain circumstances, however, they 

 all vibrate in one and the same plane, in which case 

 the ray exhibits certain peculiarities, and is said to be 

 polarised. 1 Certain crystals have the power of polaris- 

 ing such rays of light as pass through them. A few, 

 at the same time, separate each ray of light into two 

 rays which move separately from the original ray. 

 Such crystals are called double-refracting bodies. Ice- 

 land spar or, as it is also called, double spar, is the best- 

 known example of such a double-refracting body. 

 Briicke has shown that of the two substances which 

 form the alternate layers of striated muscle, the one 

 transmits light unchanged, the other is possessed of 

 double-refracting powers. But, as has already been 

 said, the contents of a living muscle-fibre must be re- 

 garded not as solid but rather as fluid, or at least as 

 semi-fluid; and observations made on living muscle- 

 fibres show that the streaks are not incapable of modi- 

 fication in their breadth and in their distance from 



1 This circumstance is treated in more detail in Lommel's The 

 Nature of Light (International Scientific Series, Vol. XVIII.) 



