CHAP. I.] 



THE PROTEIDS. 



eye-piece tube contains a Nicol's prism as analyser at K (Fig. 3), 

 and the whole tube, with the vernier and reading lens L attached^ can 



Fig. 3. LAURENT'S POLARIMETRE. 



be rotated by the screw 6r, or the eye-piece and vernier remaining fixed 

 the analyser can be rotated independently by the tangent screw F. 

 The vernier moves against the circle c, of which the rim is gradu- 

 ated. When the rotatory power of any substance is to be deter- 

 mined, a tube containing water is first placed in the position T (Fig. 4) 

 and by means of the screw G (Fig. 3) the zero of the vernier is brought 

 to coincide with that of the scale. When the eye- piece has been ad- 

 justed so that the line dividing the two halves of the field is perfectly 

 clear and sharply defined, these two halves are brought to the same in- 

 tensity by means of the screw F, the scale still reading zero. Should 

 the illumination of the field be too faint it may be increased by 

 moving the lever J (Figs. 3 and 6) slightly, though it is preferable to 

 work with the instrument when the lever is in such a position that 

 almost all the light is cut off. The water tube is now replaced by that 

 containing the substance to be tested. If it is active the two halves 

 of the field will at once be seen to be of unequal intensities. The 



