OPTICAL ACTIVITY 127 



strip of metal is given a twist, then booiv two will have to he turned 

 through a corresponding angle before the metal will slip through 

 its pages. The rotation of book two may be taken as an index 

 of the twisting of the plane of the metal strij). Various factors 

 may modify this twisting : 



{a) The nature of the metal. The same twisting force would 

 produce very different results in, say, copper and steel. 



{h) The length of the strip exposed to the twisting force. The 

 longer the strip between B^ and B^ the greater will be the twisting, 

 other conditions being equal. 



(c) Temperature. Increase in temperature will increase the 

 twisting. 



{d) Obviously the nature and strength of the distorting force 

 will modify the angle of rotation of the strip. 



A polarimeter is a device in which these basal facts arc applied 

 to light. If a beam of light (at A) is made to take the place of 

 the metal strip and for the books we substitute some optical 

 arrangement which will allow light vibrating in one plane only 

 to pass, then the eye (at C) would see a lighted field when B-^ 

 and 1?2 were in the same plane, and only then. As we shall see 

 presently, the plane in which polarised light vibrates may be 

 twisted by the action of various crystals and of several substances 

 in solution and when fused. The amount of the rotation depends 

 on the factors enumerated above, viz. : 



{a) The nature of the light. The angle of rotation depends 

 on the wave-length of the light ; the shorter the wave length the 

 greater the rotation. 



(6) The length of beam exposed to the optically active material. 



(c) Temperature as above. 



(f/) I. Nature of the optically active material : each such has a 

 specific rotatory power. 



II. Strength of solution ; double the concentration produces 

 double the rotation. 



The modification of a prism for producing light vibrating in one 

 plane was devised by Nieol and so bears his name. He made use 

 of a property of Iceland spar (calcium carbonate), namely, its 

 double refraction. Iceland spar crystallises in many forms, but 

 they are all split most readily along certain planes which are all 

 inclined to each other at fixed angles, and by cleavage the crystals 

 can always be reduced to the rhombohedral form. If such a 

 crystal of Iceland spar be placed on a piece of paper in the centre 

 of which a black dot has been made, on looking down through the 

 crystal, two ])lack dots will be seen. If now the crystal be rotated 

 without lifting it from the paper, one dark spot will remain 



