THE MICROSCOPE AS DETECTIVE 



microscope. They are easily obtained and as easily 

 observed. Each variety of starch has grains which 

 are remarkably constant in their characteristics. A 

 beginning may profitably be made with potato 

 starch, for its grains are large and they possess 

 certain well-marked features, which may or may not 

 be present in the grains of other starches. By scrap- 

 ing the newly cut surface of a potato we can obtain 

 thousands of starch grains. The surface of the 

 potato must not be grated, just a gentle scraping 

 with a pocket knife and a mere speck of the cloudy 

 liquid that is obtained, added to a drop of clean 

 water on our slide, will sufiice. Cover the object 

 with a cover glass and examine under a fairly high 

 magnification. There are countless, oval, almost 

 transparent bodies in our field of view, they are 

 potato starch grains. Each one, as we shall see 

 when we make a more careful examination, is not 

 unlike a miniature oyster-shell. In the shell, there 

 is a point which is its oldest part and the remainder 

 has grown, layer by layer, round that point till the 

 shell is fully formed. Now we magnify the starch 

 grains as highly as possible and slowly rotate the 

 fine adjustment to and fro, for the reason that the 

 object is not flat and by doing so, we obtain all its 

 parts in focus in turn. If the illumination is not too 

 intense, we shall notice a minute dark dot corre- 

 sponding to the oldest part of the oyster shell; it 

 is, in fact, the oldest part of the starch grain. 

 Around this point we can see as we focus up and 

 down, ring after ring where the grain has grown 



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