CANE SUGAR. 



20 cm. ; actually, the lamp should he so placed that an image of the source of 

 light is formed "by the illumination lens on the analyser diaphragm. This 

 position can he easily determined ; a pencil point is held immediately in front 

 of the source of light, and a piece of paper in front of the diaphragm ; at 

 the correct position a sharp image of the pencil point is formed on the paper. 



In most text hooks it is recommended that the polariscope he used in a 

 dark room, and sometimes that the source of light be in a separate room, a 

 narrow slit being cut in a wall of the room. In general this scheme is 

 inconvenient, and confinement in such a place is, in the tropics, objectionable 

 The writer finds that extraneous light may readily be excluded from the eye 

 by the following device : A piece of copper is beaten out into the form of the 



440 



