14 PRACTICAL BOTANY 



are two adjustments of focus the coarse and the fine : the latter 

 is never to be used until the focus is approximately found with 

 the coarse adjustment. The coarse adjustment is effected by a 

 sliding tube in the smaller microscopes in general useby students : 

 having drawn this tube out, screw on the low power objective 

 (the one with the larger front lens, focal length about I inch 

 from object), then replace the tube so that the objective is about 

 i^ inches from the stage, and having adjusted the mirror so as 

 to illuminate the whole field, place some object on a slide at the 

 centre of the stage ; hold the slide with the thumb and fore- 

 finger of the left hand, while the upper end of the tube is grasped 

 with the right : then slide the tube gradually downwards with a 

 spiral movement, until the object comes dimly into view ; then 

 begin to use they?;^ adjustment, which is worked by a screw 

 with a milled head, the position of which varies in different in- 

 struments : this head is to be turned so as to lower the tube, 

 when the object will become clearer, and ultimately be in perfect 

 focus. The focus is to be found in the same way with the high 

 power, but in this case greater care is necessary, since when in 

 focus the objective is nearer to the object : in careless hands 

 the position of focus is apt to be over-stepped, and the objective 

 advanced so as to touch, or even crush, the object : this is to be 

 carefully avoided, as it not only damages the object, but may 

 also ruin the objective. 



When the focus has been found, the fine adjustment is to be 

 worked constantly up and down by the right hand during obser- 

 vation ; by this means a series of optical sections of the object 

 is brought successively into view, and in this way the observer 

 builds up mentally a conception of the object as a solid body. 

 In so far as an objective lends itself to this it is said to possess 

 good penetration. Meanwhile the forefinger and thumb of the 

 left hand will be at liberty to move the slide on the stage so as 

 to bring into the field of view different parts of the preparation. 



Observers should accustom themselves to using both eyes 

 indifferently, and when one eye is being used for observation, 

 the other should be kept open : a little practice will soon over 

 come any difficulty which may be at first found in doing this. 



Care is necessary in removing the slide from the stage 



