CHAP. IL] THE CONTRACTILE TISSUES. 93 



opposite the intermediate line, and bulges out in the interval 

 between each two intermediate lines, the bulging and indentation 

 becoming more marked the greater the contraction. 



56. We can learn something further about this remarkable 

 change by examining the fibre under polarized light. 



When ordinary light is sent through a Nicol prism (which is a 

 rhomb of Iceland spar divided into two in a certain direction, the 

 halves being subsequently cemented together in a special way) it 

 undergoes a change in passing through the prism and is said to be 

 polarized. One effect of this polarization is that a ray of light which 

 has passed through one Nicol prism will or will not pass through a 

 second Nicol according to the relative position of the two prisms. 

 Thus if the second Nicol be so placed that what is called its " optic- 

 axis " be in a line with or parallel to the optic axis of the first Nicol 

 the light passing through the first Nicol will also pass through the 

 second. But if the second Nicol be rotated until its optic axis is at 

 right angles with the optic axis of the first Nicol none of the light 

 passing through the former will pass through the latter; the prisms 

 in this position are said to be 'crossed.' In intermediate positions 

 more or less light passes through the second Nicol according to the 

 angle between the two optic axes. 



Hence when one Nicol is placed beneath the stage of a microscope 

 so that the light from the mirror is sent through it, and another Nicol 

 is placed in the eye-piece, the field of the microscope will appear dark 

 when the eye-piece Nicol is rotated so that its optic axis is at 

 right angles to the optic axis of the lower Nicol, and consequently 

 the light passing through the lower Nicol is stopped by it. If however 

 the optic axis of the eye-piece Nicol be parallel to that of the lower 

 Nicol, the light from the latter will pass through the former and the 

 field will be bright ; and as the eye-piece is gradually rotated from one 

 position to the other the brightness of the field will diminish or 

 increase. 



Both the Nicols are composed of doubly refractive material. If 

 now a third doubly refractive material be placed on the stage and 

 therefore between the two Nicols, the light passing through the lower 

 Nicol will (in a certain position of the doubly refractive material on 

 the stage, that is to say when its optic axes have a certain position) 

 pass through it and also through the crossed Nicol in the eye-piece. 

 Hence the doubly refractive material on the stage (or such parts of it 

 as are in the proper position in respect to their optic axes) will, when 

 the eye-piece Nicol is crossed, appear illuminated and bright on a dark 

 field. In this way the existence of doubly refractive material in a 

 preparation may be detected. 



When muscle prepared and mounted in Canada balsam is 

 examined in the microscope between Nicol prisms, one on the 

 stage below the object, and the other in the eye-piece, the fibres 

 stand out as bright objects on the dark ground of the field when 

 the axes of the prisms are crossed. On closer examination it is 

 seen that the parts which are bright are chiefly the dim bands. 



