212 



PRACTICAL PHYSIOLOGY 



but in this instrument itself it is found more convenient to use a polariser and 

 analyser made of a Nicol's prism. A Nicol's prism consists of a crystal of 

 Iceland spar. Such a crystal has the power of splitting light into two rays, 

 one of which, the ordinary ray, passes through it as it would through glass, 

 and the other one, the extraordinary ray, is more refracted. Consequently, 

 on looking at a dot on a sheet of paper through a piece of Iceland spar laid 

 flat on the paper, a double image of the dot is obtained, and if the crystal be 

 rotated, one of the dots the extraordinary ray will be seen to move round 

 the other the ordinary ray which remains stationary. Now both these rays 

 are polarised, but in different planes. If the crystal be cut across along a 

 diagonal line and the two surfaces re-cemented by means of Canada balsam, 

 the ordinary ray, when it meets the balsam, will be totally reflected and pass 

 out at the side of the crystal, whereas the extraordinary ray will be trans- 

 mitted through the balsam, and will finally emerge at the end of the prism, 



FIG. 174. Polarimeter of Mischerlich with Laurent's polariser. 



P, polariser and device for obtaining half shadow ; R, fluid container ; T, scale with vernier c 

 attached to pointer; A, compensator and analyser; F, lens. 



parallel to its original direction ; but, of course, plane polarised. To 'detect 

 the polarisation a similarly constructed prism, or analyser, must be used. 



Certain other bodies, e.g. a quartz plate, a solution of sugar or albumin, 

 have the power of rotating the plane of polarised light. Thus, supposing that 

 the plane polarised light vibrates along a vertical plane, one of these bodies will 

 cause it to vibrate in an oblique plane. If the analyser be so placed that none 

 of the plane polarised light can pass through it (i.e. the field is black), and if a 

 piece of quartz be inserted between the polariser and analyser, it will be found 

 that now a certain amount of light passes through the analyser (i.e. the field 

 becomes opaque), and, in order to obtain darkness again, it is necessary to 

 rotate the analyser in the direction of the hands of a watch, as seen by the 

 observer. Consequently, rotation has taken place to the right, i.e. dextro 

 rotation is said to have occurred. If a solution of albumin or Isevulose be 

 employed the rotation of the analyser must be to the left, i.e. against the 

 hands of the watch. When the plane of white light passes through the quartz 



