BY DR. KLEIN. 77 



that all those parts which are isotropous are invisible. If, 

 however, the field is colored, by placing a plate of mica or 

 selenite underneath the object, everything is seen as distinctly 

 as if the light were not polarized, with the difference that the 

 doubly refractive bodies are distinguished from others by their 

 color the latter being" of the color of the field, the former of a 

 color differing from it variously, according to their thickness, 

 their position, and their optical properties. In all doubly re- 

 fracting ciystals, there is at lenst one direction in which light 

 may be transmitted without suffering double refraction i. e., 

 bifurcation. Those crystals in which there is only ono such 

 direction are called uniaxial, e. <?., Iceland spar, quartz, and 

 tourmaline. When such crystals are examined between crossed 

 Nicols, in such a position that the light is transmitted through 

 them in the direction above referred to (which is always that 

 of the axis of crystallization), they are not seen. We shall lind 

 that the same holds good as regards the anisotropous parts 

 of muscular fibre. 



In a fresh muscular fibre seen between crossed Nicols, the 

 first fact that strikes one is that the appearances correspond 

 with those observed in doubly refracting bodies. Next it is 

 seen that all tire muscular fibres under observation are not 

 equally illuminated. Those are brighest which are so placed 

 that the long axis forms an angle of 45, the illumination di- 

 minishing as the angle diminishes, until it disappears at the 

 moment that the fibre axis lies in the plane of polarization of 

 either Nicol. It is further seen that all parts of the muscular 

 fibre are not doubl}' refracting, but only those parts which 

 were before described as sarcous elements. The interstitial 

 substance looks dark whatever be the position of the fibre, so 

 that between crossed Nicols it is invisible. 



The method to be adopted foi demonstrating these facts is 

 as follows : 



Method. From a number of plates of selenite or mica, one 

 is selected which, when placed in the proper azimuth, gives be- 

 tween crossed Nicols a field of the tint which is known as 

 teinte de passage. 1 Such a plate having been found, it is fixed 

 to the object-glass with a drop of dammar. Fresh muscular 

 fibres of the extremities of the crab, frog, or hydroplnlux, are 

 placed in absolute alcohol for half an hour, or in ordinary al- 

 cohol for several days. As soon as the muscular tissues are 

 deprived of water, they are soaked in oil of turpentine. Of the 



1 The teinte de passaged a peculiar purple violet, and lies between 

 red and blue in this sense, that if the plate possess a thickness a shade 

 greater than that which produces the tint required, the color is blue ; if 

 a shade less, red. These facts are of importance as aids in selecting 

 a plate. 



