10 T. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



We will now commence our work. An object-slide or 

 micro-slide is wiped clean with a soft cloth, and upon it, by 

 means of a thin glass rod, a drop of pure or distilled water is- 

 placed. 



Making a Preparation of Potato Starch. Let us commence 

 our study by the aid of a potato-tuber. We cut this through with 

 a pocket knife, and transfer a little of the sap which exudes from 

 the cut surface into the drop of water by means of the same knife. 

 We then cover the drop with a cover-glass. This must also have 

 been previously cleaned with special care, which is best done flat 

 between the fingers by the aid of pieces of old fine linen or soft 

 Japanese writing-paper. This operation is not so simple as it 

 seems. If the cover-glasses are thin, they are very easily broken. 

 A method I [Eo.] have proved for many years to be least destruc- 

 tive for beginners is to slightly damp the thumb and index finger 

 of one hand, say the left, place over the hand a piece of linen 

 or silk, and push it in between the thumb and finger so as to form 

 a fold. Taking the cover-glass by its edges between the thumb 

 and finger of the other hand, slip it horizontally into the fold of 

 the fabrics and bring the covered thumb and finger down upon it 

 one above and the other below and rub them to and fro on its- 

 two surfaces. The fabric will cling to the slightly damp fingers, 

 and the process becomes easy. Some use little pads between 

 which the cover-glass is placed, and the pads then moved about 

 over its surface. The cover-glass must be laid on the preparation 

 as carefully as possible, so as to exclude air from underneath 

 it. For laying on, it can be held between the index finger above, 

 and a needle (in a holder) underneath. By pressing the thumb-nail 

 against the end of the needle-holder, the latter can be gradually 

 withdrawn when the cover touches the water, and this is gently 

 lowered into its place. If the drop is of proper size no water will 

 flow out from the side of the cover-glass. The size of the drop 

 has usually to be estimated in respect to (1) the size of the cover- 

 glass, and (2) the thickness of the preparation to be covered ; but 

 in this present case this latter is very small. If water does flow 

 out it can be removed with blotting-paper ; or it is better, indeed, 

 to make a second preparation, as most of the grains which we 

 wish to observe would be sucked out by the blotting-paper. 



Using the Microscope. We now place our preparation on the 

 stage of the microscope, so that the object lies over the centre of 

 the opening in the stage, and place the low power in position for 



