558 



A MANUAL OF PHYSIOLOGY 



Appearance of the Fibres in Polarized Light. A ray of ordinary 

 light consists of vibrations of the ether in all planes at right angles 

 to the direction of the ray. In a ray of plane polarized light all the 

 particles vibrate in one plane. A ray of light which has been polarized 

 by a Nicol's prism cannot pass through another Nicol's prism with 

 its principal plane at right angles to that of the first. If the second 

 or analyzing prism be rotated so that the principal planes are no 

 longer at right angles, some of the light will pass through. The 

 same effect is produced if, without altering the original ' crossed ' 

 position of the nicols, a substance capable of rotating the polarized 

 ray is introduced between the prisms. A rough illustration will 

 perhaps tend to make this point clearer. Suppose that a string fixed 

 at one end is set vibrating in various directions by a twisting move- 

 ment. If the string has to pass through a narrow vertical slit, e.g., 

 between two fingers held vertically, all vibrations except those in the 

 vertical plane will be extinguished; but vertical vibrations will be 

 able to pass beyond the slit. The movement may be said to be 

 plane polarized, and the effect of the slit corresponds to that of the 

 first nicol. Now make the string pass also through a horizontal slit ; 

 the vertical vibrations will then be extinguished too ; in other words, 

 none of the movements will pass beyond the * crossed ' slits. This 

 corresponds to the dark field of the crossed nicols. But if the vertical 

 vibrations which have passed the first slit could be in any way 

 changed into horizontal vibrations, they would no longer be extin- 

 guished by the second. This would correspond to rotation of the 

 plane of polarization through 90. A ray of light polarized by the 



first nicol will, if its plane of 

 polarization be rotated through 

 90, pass entirely (except for 

 'oss by ordinary reflection and 

 absorption) through the 

 second. If the angle of rota- 

 tion is less than 90, a portion 

 will pass through. 



The substance of the Bow- 

 man's element, and particu- 

 larly the easily-stained material 

 in it, is doubly refracting, and 

 therefore rotates the plane of 

 polarization. The same is true 

 of the Dobie's globule, but 

 the rest of the intermediate 

 segment is singly refracting. 

 When an uncontracted fibre 

 is viewed with crossed nicols, 

 the dim stripe accordingly ap- 

 pears bright in the otherwise dark field. In the contracted fibre 

 the stripe that is dim in ordinary light is bright when looked at with 

 crossed nicols, since the ends of the Bowman's elements, filled 

 with the doubly refractive stainable material, and the doubly re- 



FIG. 166. LIVING MUSCULAR FIBRE 

 (FROM GEOTRUPES STERCORARIUS). 



i, in ordinary ; 2, in polarized light. (Van 

 Genuchten.) In living muscle (at least in 

 fibres which are not extended) in contrast to 

 dead muscle after treatment with reagents, 

 the doubly refracting or anisotropous sub- 

 stance is present in the greater part of the 

 fibre ; and with crossed nicols the position of 

 the singly refracting or isotropous material is 

 indicated only by narrow transverse black 

 lines or rows of dark dots. 



