PHYSIOLOGICAL CHEMISTRY 281 



light is capable of vibrating in one plane only, which would correspond, in our 

 example, to two opposite spokes. The light which vibrates in one plane is 

 called plane-polarised light, and cannot be distinguished by the naked eye 

 from ordinary light. By placing a second, similarly cut, tourmaline plate in 

 its course, however, it can be detected, for it will pass through this only if its 

 axis corresponds to the axis of the first plate. The first plate is called the 

 polariser and the second plate the analyser. The mechanism of this action of 

 the analyser and polariser can be easily illustrated by a piece of string stretched 

 between two posts ; it can vibrate in all planes. If a comb be placed in the 

 course of the string the vibrations can only take place along one plane corre- 

 sponding to the direction of the teeth of the comb. This comb represents the 

 polariser. If now, a second comb be placed along the string it will permit the 

 vibration of the string or stop it, according to the position of its teeth ; if 

 these be in the same direction as those of the first comb the string will go on 

 vibrating, but if they be placed at right angles the string will cease to vibrate. 

 Polarisation of light by tourmaline illustrates the principle of the polarimeter 

 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, 

 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 laevulose 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 

 plate, however, the various colours of the spectrum are rotated to a different 

 degree, so that, instead of having a mere opacity (as is the case with inter- 

 mediate positions of two ' tourmaline ' plates) different colours are obtained 

 according to the amount of rotation. There are also samples of quartz which 

 rotate the plane of light to the left. 



