THE ACTION OF AMYLASE ON STARCH 109 



field appearing equally illumined (equal half -shadows). For 

 this purpose the telescope F is focussed on the quartz plate, 1 

 so that the field presents a perfectly clear round circle divided 

 into two equal parts by a sharply defined vertical line. If 

 the graduated dial is turned through three or four degrees to 

 either the right or the left of the zero line, it will be seen that 

 one half of the field will become lighter, the other half darker. 



In the first place, the zero position is so adjusted that the 

 zero line of the circle coincides with the zero line of the 

 vernier. The half -shadow can now be made lighter or darker 

 (according as the polariser is turned to the right or left of the 

 zero line) by means of the pointer reaching from the dial 

 segment. When the pointer h is in the zero position, and at 

 the same time the analyser A is placed in the zero position, 

 both halves of the field of view appear black. The nearer 

 the pointer is to the zero line, the darker the half-shadow will 

 become, and the more sensitive the apparatus ; but when 

 the solutions are not quite transparent, the pointer must be 

 moved more or less away from the zero line, so that the field 

 is clear. For the majority of experiments the position of the 

 pointer at 7| is most suitable, therefore the apparatus is 

 usually so adjusted that in this position the dial and vernier 

 read exactly 0. When the pointer is moved, of course the 

 zero point of the apparatus changes, and no longer corre- 

 sponds with the zero line of the dial. The difference between 

 the latter and the zero position of the apparatus must either 

 be taken into account (the simpler way), or else after the 

 graduated dial has been moved to the apparatus must be 

 again placed in the zero position : to do this, the analysing 

 Nicol prism is turned, by means of the screws A, to the right 

 or left until the half-shadows are equal in tint. 



Special attention must be called to the following circum- 

 stance, which, if not noted, may lead to considerable confusion. 



When the circle has been turned too far, and has gone 

 1 Or small Nicol prism in the Lippich model. 



