22 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



more take potato starch, place a drop of the reagent (5-0 or lO'O 

 per cent, solution) against the edge of the cover-glass, and await 

 its entrance. The action must take place quite gradually, if it is 

 to be instructive. We then notice, at the first moment of the 

 action, that the lamination shows up more clearly ; quickly, 

 however, disappearing, while the grain increase in size. During 

 this enlargement, which proceeds with more or less regularity, 

 the hilum of the starch grain hollows out considerably, upon 

 which the wall of the weaker side, that towards the anterior end 

 of the grain, sinks into the hollow. Later on the regularity of 

 the phenomenon disappears altogether, and the grain enlarges 

 into a mass of considerable volume, and as clear as glass, the 

 limits of which are ultimately scarcely recognisable. In some 

 respects it is an advantage in this reaction to use one of the 

 preparations which has been stained pretty deeply with iodine. 

 The potash destroys the colour, but it persists long enough to 

 enable the swelling of the grains to be more easily followed. 



Swelling by Heat. Lastly we can endeavour by warming the 

 preparation to cause the starch to swell, a method which is, 

 indeed, in use in making paste. The preparation is warmed over 

 a spirit or gas flame, without allowing it to boil, and taking care 

 to replace by fresh water that which evaporates. If a tempera- 

 ture of about 70 C. (160 F.) is reached, the grains will be found 

 to be swollen just as by the potash method. In this reaction, 

 also, it is of advantage to treat the starch previously with iodine. 

 According to the extent of heating the colour fades or disappears, 

 though it may reappear on cooling. In order to determine with 

 accuracy the temperature at which starch swells, the preparation 

 must be warmed upon a heating microscope stage. 



Polarisation. If a polariscope should happen to be at our 

 disposal it will enable us to get an instructive insight into the 

 character and physical structure of starch grains. A polariscope 

 consists of two parts, known as polariser and analyser respectively, 

 each of which consists essentially in a prism of Iceland spar 

 (" Nicol's prism ") ; the former is placed below the stage so that 

 the light passes through it before entering the object. The latter 

 is placed above the object, being either interposed between 

 objective and eye-piece, or passed over the latter like a cap. If 

 a preparation of potato starch be placed on the stage in polarised 

 light, i.e., with the prisms of the analyser and polariser crossed 

 by partially rotating one of them, the field of view is darkened, 



