THE POLARIMETER 



487 



appliance, it is convenient to arrange a cradle alongside the trough of the 

 polarimeter to hold the Pellet tube whenever an ordinary tube is brought 

 into use. 



The tubes used for materials when temperature control is important, as 

 in the reading after inversion, are water-jacketed and are supplied with a 

 tubulure for the insertion of thermometer and stopper, as indicated in Fig. 



321. 



Tubes are found made of both glass and metal. The former must be 



Fig. 320 



used for acid materials and is preferable on the grounds of smaller expansion. 

 In addition, metal tubes ma\' become bent, due to rough use, without the 

 damage being observed. The life of a glass tube is shorter than that of a 

 metal tube, but fracture is only due to avoidable carelessness. 



Polarimeter tubes are supplied in lengths of 2.5 cms., 5 cms., 10 cms., 

 20 cms., 22 cms. (for elimination of calculation in certain routine dilutions) 

 40 cms. and 60 cms. The Laurent instruments are usually built to accom- 

 modate, and are supplied with, a 50-cm. tube. 



The diameter of a tube should be larger than that of the diaphragm 

 through which the pencil of light passes, so as to avoid depolarization due to 

 internal reflection, and the glare which accompanies a tube of smallei" 

 diameter. Bates adopts g mm. as a convenient diameter. 



Fig. 321 



Convenience in Observation. — A dark room, or cabinet enclosing the 

 instrument, with source of light located externally is usually advised. In 

 place thereof the writer finds the use of a shield of the form shown in Fig. 322 

 very effective to cut off extraneous light. 



Polarimeter Scale. — The scale of the polarimeter is usually mounted on 

 the moving wedge of the compensator. The vernier is stationary. The 

 scale is either made of some alloy as nickelin, the expansion of whicli is low, 

 or of invar, the expansion of which is zero. In some patterns the scale is 



