FOOD DIGESTION AND RESPIRATION 49 



their behaviour to polarised light. A beam of ordinary light 

 consists of a number of ether elements vibrating in all possible 

 directions at right angles to the direction of the beam. If looked 

 at endwise, these vibrations fill up, as it were, the whole cross 

 section of the beam, in all directions across it. There are, however, 

 certain crystals which, owing to their structure, only allow vibra- 

 tions of one particular direction to pass through, the others being 

 blocked out or absorbed. The same thing can also be done by 

 reflection from glass at a particular angle, in which case all the 

 vibrations except those of one direction pass through ; those of 

 the particular direction are reflected. The beam is then said 

 to be " polarised," because it has properties different in one 

 direction from those in another direction. Suppose such a beam 

 to be sent through a compound which contains asymmetric carbon 

 atoms. Owing to their being different in one direction from 

 that in another, such atoms will turn the plane in which the 

 polarised light is vibrating through a certain angle. They are 

 said to rotate the plane of polarised light, and to be "optically 

 active." Now, according to the side of the carbon atom which is 

 the more heavily weighted, the plane of the polarised light 

 will be turned either to the right or to the left. Hence the two 

 kinds of " optical isomers," as we may call them now, can be 

 distinguished. That particular form of glucose, which is the only 

 one utilised by the organism, rotates to the right ; that form of 

 alanine used rotates to the left. The means used for the detection 

 and measurement of the degree of this rotation of polarised light 

 is the instrument called the " polarimeter " (E., p. 181). The prin- 

 ciple of it is this : light sent through the instrument is first 

 polarised by a prism of Iceland spar cut in a particular way. At 

 the eye end there is another similar prism which can be rotated, 

 and the angle of rotation measured. If the plane of vibration 

 of the light passed by the polarising prism is the same as that 

 passed by the second prism (" analyser "), the light reaches the eye. 

 If not, there is darkness. When a solution of an optically active 

 substance is placed between the prisms, the analysing prism 

 requires rotation in order to correspond with the plane of vibration 

 of the light which has been rotated by passing through the 

 solution. In the actual instrument there is a device which 

 increases its sensibility, so that very small differences of rotation 

 can be measured accurately. 



It appears that living organisms must have first made their 

 appearance under the influence of some asymmetrical forces, so 

 that they developed a bias towards one set of optical isomers. 

 Once established, this would tend to become more and more 

 exaggerated. The question is a difficult one, but it must not be 



