BEHAVIOUR OF MUSCULAR FIBRES IN POLARISED LIGHT. 237 



surface remains constantly dark in the dark field of the crossed 

 Nicol's prism, whilst the remainder, in the effective azimuths 

 that is, in those in which they make angles between and 45 

 with the planes of polarisation become clear. It soon appears 

 that such as always remain dark are those which lie exactly 

 parallel to the axis of the instrument, whilst this is not the 

 case with the rest. There is thus an optic axis precisely corre- 

 sponding with the longitudinal direction of the muscular fibres. 

 Now, inasmuch as this coincides with the longitudinal dimen- 

 sions of the straight prisms represented by the sarcous elements, 

 and since we are unable to discover a second optic axis, or 



Fig. 38. 



any indication of its existence, we must regard the sarcous 

 elements as uniaxial. 



Are they positively or negatively doubly refractile ? In 

 order to determine this I have constructed the instrument 

 shown in the accompanying figure. The blackened brass 

 plate a a, perforated in the centre, and connected to the 

 object plate of the microscope, possesses two slides, which 

 can be moved over one another ; the lower c c by means of 

 the micrometer screw 6, the upper e e with the unassisted 

 hand by means of the handle d on the parallelogram g g. 

 Both slides carry prisms of quartz, the upper one movable 



T2 



