76 MUSCULAR TISSUE. 



Before proceeding to describe what is seen in muscle when 

 examined between crossed Nicols, the facts observed when 

 crystals which possess similar optical properties are looked at 

 in the polarizing microscope, should be first carefully studied. 

 Muscular fibres can be shown to possess optical properties which 

 resemble thqse of doubly refractive, positive, uniaxial crystals,' 

 such, e. </., as those of rock crystal or quartz, etc. The meaning 

 of these expressions must be illustrated. If a number of doubly 

 refracting microscopical crystals of any kind are placed under 

 the polarizing microscope, it is seen that when the upper Nicol 

 (or analyzer) is rotated so as to make the field dark, the crys- 

 tals appear (according to their position) more or less illumin- 

 ated ; whereas this is not the case if the crystals are isotropous, 

 i. #., belong to the " regular" system of crystallization. 



The degree of illumination of each crystal varies according 

 to its position. This may be readily shown by rotating the 

 object-glass, or stage on which it lies, without moving either 

 prism : it is then seen, as regards each crystal (supposing the 

 Nicols to be crossed), that four times in each complete rotation 

 it loses its luminousness altogether. These two positions are 

 called the inactive azimuths, because the body looks in these 

 positions as if it were isotropous dark on the dark field. 

 This happens whenever the principal plane of the crystal lies 

 in the principal 'plane of either Nicol, and is consequently at 

 right angles to that of the other. In all other positions it looks 

 more or less illuminated, the degree of brightness increasing 

 and diminishing as the azimuth in which it is placed declines or 

 approaches; consequently, the crystal appears brightest when 

 its principal plane is inclined at an angle of 45 to the plane of 

 polarization. When the crystalline body is of a certain thick- 

 ness, the appearances are somewhat different. Thus, if a plate 

 of mica from one to two millimetres thick is placed on the ob- 

 ject-glass with the Nicols crossed, it is seen that the field is not 



, only luminous but colored, the color varying according to the 

 thickness of the plate its intensity varying according to the 



' inclination of the principal plane of the mica to that of the 

 prisms, being brighest when that inclination is 45. If now the 

 plate of mice is rotated, it is seen that in each rotation, as be- 

 fore, there are four azimuths of greatest brightness, and four 

 intervening ones of greatest obscurity. Cut, in addition to 

 this, it is observed that, in the bright azimuths, the colors dis- 

 played differ the color of the field in any given position of the 

 plate being complementary to that seen when it is rotated 90. 

 These facts are of great practical importance in all cases in 

 which it is desired to observe the doubly refractive parts of 

 transparent objects between crossed Nicols, without losing 

 sight of those parts which are isotropous. If such objects are 

 examined in the ordinary way in the dark field, it is obvious 



