54 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



chains can be determined unequivocally in even the smallest micro- 

 scopically visible piece of wall, and the actual value of the refractive 

 indices gives some information (commonly rather difficult to interpret, 

 however) concerning irregularities. Investigation of wall structure by 

 optical methods involves, therefore, directly or indirectly, the deter- 

 mination of refractive indices. For many purposes it is, as a matter of 

 fact, unnecessary actually to determine the value of the indices but 

 sometimes the information to be sought can be obtained only by careful 

 measurement. The method involved may therefore briefly be described 

 before passing to the commoner methods employed. 



The measurement of refractive indices 



In principle the method used for objects of microscopical size is a 

 very simple one. It involves nothing more than finding a liquid medium 

 in which there is no bending of the rays of light at the edge of the fibre, 

 so that the fibre becomes almost invisible. It never does in practice 

 become completely invisible for a number of reasons into which we need 

 not go at the moment. Since the refractive index varies with the wave- 

 length of the light used, it is customary to use monochromatic radiation 

 given by a sodium vapour lamp, i.e. to use the sodium D lines. The 

 method can be tedious, but if a set of liquids is prepared beforehand 

 differing in refractive index by, say, 0-01, and a sufficient quantity of 

 material in an adequate condition is at hand, then results come more 

 quickly than might be imagined. 



As a guide to the choice of the correct liquid a phenomenon first 

 described by Becke and called the Becke Hne is employed. If the boun- 

 dary between two media is observed under a microscope then a line of 

 light is visible along the boundary if the refractive indices of the media 

 are different. On raising the microscope objective, this line of light 

 moves into the medium of higher refractive index. It is, therefore, a 

 matter of a moment to decide whether the immersion liquid bathing a 

 fibre has an index lower or higher than that of the fibre and thus to 

 obtain a choice for the next liquid to be tried. The method, then, is 

 briefly this. The fibre is mounted in a suitable medium of known refrac- 

 tive index on the rotating stage of a polarizing microscope. Below the 

 condenser of the microscope is a Nicol prism, or some corresponding 

 polarizing device, and this is left in position. A second Nicol, either in 

 the body tube or the eyepiece depending on the design of the micro- 

 scope, is thrown out of the optical system, after using the combination 

 of the two prisms, by a method to be described later, to set the fibre with 

 the direction of, say, the higher refractive index parallel to the direction 



