POTATO STARCH AIR BUBBLES. 15 



some cases disappearing altogether. At this more feebly developed 

 -end of the grain, which we will call the anterior end, the layering 

 is indistinct the individual grains vary considerably in size, and 

 deviate, moreover, from one another in outer form to a not incon- 

 siderable extent, and show the layering with various degrees of 

 sharpness. In the smallest grains the layering is hardly recog- 

 nisable. 



Between the starch grains in most preparations will be found 

 rounded bodies, which, with a median focus, show a small, round, 

 bright centre, and a broad, dark margin ; this last is black at its 

 inner edge, dark grey outwardly, and interrupted by clear inter- 

 mediate rings. These structures are air bubbles, enclosed in the 

 mounting fluid. Their appearance under the microscope is so 

 -characteristic that, once known, they can scarcely ever be con- 

 fused with other appearances. The rays of light which pass out 

 of the denser medium into the air bubble are, with the exception 

 of the central ones, so strongly refracted (or bent) that, they 

 cannot pass into the objective, and hence the broad dark rim, 

 and the comparatively small bright centre. If by turning the micro- 

 meter screw right-handedly (as in driving a screw into wood), 

 the tube i lowered, i.e., " focussed down," so that the under part 

 of the air bubble comes into view, the sharpness and brightness 

 of the middle disk increases ; it diminishes at the same time in 

 size, while the breadth of the surrounding dark ring increases. If 

 the screw is moved in the opposite direction, or " focussed up," in 

 order to focus upon the upper part of the air bubble, the middle 

 disk enlarges, but loses somewhat in brightness; a grey ring of 

 varying degree of brightness arises round it ; the surrounding 

 rim becomes at the same time narrower. You may possibly see 

 in the preparation a dark band completely crossing the field of 

 view, which represents the rim of a very large patch of air, and 

 shows, mutatis mutandis, the same changes with focussing up or 

 down. 



Drawing the Object. If you have found a clearly laminated 

 starch grain it should now be drawn. In microscopical observa- 

 tion the greatest possible stress is laid upon drawing. By its aid 

 we first learn to observe acutely, and many special features of struc- 

 ture first become evident to the observer when he concentrates his 

 attention upon an object for the purpose of making a representa- 

 tion of it. Drawing, therefore, protects from cursory superficial 

 observation, enforcing thorough incisive study of the image, and 



