ESTIMATION OF CARBOHYDRATES 



FIG. 43. 



At S, lens and bichromate cell. At P, polarising prism. At h, lever to rotate polarising- 

 prism. At A, analysing prism which can he-rotated by a screw. At F, telescope with, 

 eye-piece. K = graduated scale, n, n' = fixed verniers. T = screw for rotating gradu- 

 ated scale. I = magnifying lens to read scale and verniers. 



2. The Observation Tube. 



The solution of the substance is placed in a special observation 

 tube (Fig. 44). These tubes are generally 0*5, I, 2, 2*2 decimetres 

 long ; they are made of glass of the exact length ; the ends are closed 

 by cover glasses held in place by a screw cap and rubber washer. Very 

 small tubes for use with small amounts of solution are also made. ' 



These tubes are thoroughly dried by pushing a plug of filter paper 

 through them, or thoroughly washed by rinsing several times with the 



FIG. 44. 



solution under examination. The cover glasses must be dry and 

 without serious scratches. One end of the tube is closed by a cover 

 glass, brass cap and washer, and the solution is filtered into it at the 

 other end until a meniscus just projects above the opening. A short 

 time is given to allow air-bubbles to rise. The other cover glass is 

 slid horizontally over the end of the tube so that it pushes off the 

 excess of liquid and exactly covers the end leaving no air-bubbles 



