ADJUSTMENT OF MICROSCOPE 19 



The coarse adjustment is effected by a sliding tube in the 

 smaller microscopes in general use by students : having 

 drawn this tube out, screw on the low power objective 

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

 1 inch from object), then replace the tube so that the 

 objective is about 1J 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 forefinger 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 the fine adjustment, which is 

 worked by a screw with a milled head, the position of 

 which varies in different instruments : this head is to 

 be turned so as to lower the tube, when the object will 

 become clearer, and ultimately 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 observation; 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 



c 2 



