6 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



might come from the opposite process of "focussing down". 

 Further reference to this important point will be made in our 

 study, shortly, of our first micro-preparation. 



It will be noticed that the tube of the microscope can be 

 lengthened by a draw-tube (D T). This gives increased magnifying 

 power, and with this microscope the tube may be elongated an 

 inch or so, with increased magnification of 15 or 20 per cent., with- 

 out disadvantage. It must be borne in mind, however, that the best 

 work of a double system of lenses, such as we are using, is done 

 when the distance between objective and eye-piece is that for 

 which the lenses are themselves actually constructed, and that 

 any increase or diminution of this distance is disadvantageous. 1 

 Increased magnification per se is of no advantage whatever. 

 The constant tendency of the student is to magnify up to the 

 utmost limits of his instrument. This is an error to be stead- 

 fastly avoided. That amount of magnification is best at which you 

 can see most clearly what you are looking for. Size may readily be 

 purchased at the expense of distinctness. 



One final word of warning. A microscope should always be 

 lifted by the foot, never by the stage, nor by the body above the joint. 

 In the present instrument the hind foot is admirably located for 

 grasping. 



Microscope for Higher Work. The microscope described above 

 is capable of doing satisfactory work in most departments of 

 anatomical botany. It is capable also of being improved by 

 various subsequent additions. First amongst these should doubt- 

 less be the substitution of rack and pinion coarse adjustment for 

 that by a sliding tube ; indeed for a small extra sum (1 Is.) the 

 microscope can be from the first provided with it. The opinions 

 of practical teachers on the desirability of a learner from the 

 first having a rack and pinion instrument vary greatly ; laboratory 

 instruments are rarely provided with it, but with institutions, 

 as with individuals, questions of first-cost are apt to intrude 

 themselves. My own experience [Ed.] is simply this : that, pro- 

 vided focussing up be practised, the advantages are emphatically 

 in favour of the mechanical method of movement, and this is 

 particularly the case with instruments in which the rack and 



1 With the microscope here described, as with most small stands, this 

 distance is 8 inches over all ; so that the most effective distance for use, when 

 the double nose-piece is in position, is with the draw-tube drawn out to the 

 line " 160 " engraved upon it. 



