USING THE MICROSCOPE POTATO STARCH. 13 



drawing the tube. This must be raised sufficiently far for the 

 purpose, so that the one objective can be unscrewed, and the other 

 put in its place ; this is, indeed, essential where the tube is 

 actuated by rack and pinion. The comparatively low price, 

 however, at which it is possible now to purchase a "revolving 

 nose-piece" makes this an adjunct to most instruments. 



We now begin the actual study of the micro-preparation. The 

 learner should accustom himself, so far as his eyes are equally 

 good, to observe with the left eye. The right eye is thus kept 

 free, and can be used in drawing, while he continues to observe 

 with the left eye. Many of the drawing prisms and appliances 

 for the microscope, moreover, are constructed for left-eye observa- 

 tion, and those who work with the right eye should intimate it in 

 ordering such drawing prisms. The learner should also keep open 

 the eye which is not in use. At first the surrounding objects, 

 which are figured in the retina of the eye, will disturb him ; but 

 he will soon overcome the difficulty of concentrating all his atten- 

 tion on the eye engaged in observation, and temporarily suspending 

 the activity of the other. 



Potato Starch. We readily recognise that the colourless bodies 

 which occupy the field of view of the microscope are solid, and 

 that they show lamination. They are starch grains. We slowly 

 move the object-slide about in order to find a place where the 

 grains are not too crowded, because in such a place it is more 

 easy to fix attention upon a single grain. We select for careful 

 study a grain which shows lamination with especial clearness. 

 As the movement of the object- slide under the microscope appears 

 to be reversed, we shall at first find some difficulty when we try 

 to place a particular selected grain in the centre of the field of 

 vie\v so as to examine to greatest advantage ; and we shall have 

 as quickly as possible to accustom ourselves to control the slight 

 movements upon which it depends. If we have found a specially 

 favourable grain, we may magnify it still more by now removing 

 the weak eye-piece and replacing it by a stronger, if we have one. 

 If the eye-piece fits at all tightly, hold the tube of the microscope 

 firmly while you do this, or the focussing may be altered, and the 

 objective possibly forced down on the preparation. With good 

 lenses the figure still remains good, although the brightness of it 

 and of the field is diminished. We endeavour, so far as possible, to 

 obviate this by improved adjustment of the position of the mirror. 

 The illumination can be immensely improved by means of a sub- 



