420 PRACTICAL PHYSIOLOGY 



called asymmetrical C-atoms because their substitutes might have 

 different spatial arrangements, and still the empirical formula remain 

 the same. 



This variable arrangement of the substitutes causes these slight 

 differences in the chemical behaviour of the body. It also manifests 

 itself, however, by differences in the stereochemical behaviour of the 

 bodies, and this is ascertained by examining the rotatory power on a 

 beam of polarised light. 



Polarisation of Light- When two slices of tourmaline, a semi-transparent 

 mineral, are cut parallel to the axis of the crystal and laid over one another, it 

 will be noticed that the amount of light which passes through the combination 

 varies according to the relative positions of the two slices to one another. If the 

 slices be at right angles to one another no light passes through, and in intermediate 

 positions only a certain amount, so that an opaque combination is obtained. A 

 ray of ordinary light contains vibrations in all planes passing through the ray ; 

 but, when the light passes through a tourmaline plate, it vibrates in one plane only. 

 Ordinary light may, therefore, be likened to a wheel, the axle representing 

 the ray of light and the spokes the planes along which it vibrates. On 

 passing through the tourmaline plate, however, the 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 corresponding 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 polariser, but in this instrument itself it is found 

 more convenient to use a polariser and analyser made of a Nicol's privm. 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 transmitted through 

 the balsam, and will finally emerge at the end of the prism, parallel to its original 





