120 PRACTICAL DIRECTIONS 



Immersion lenses should be used only with stained preparations. They are 

 not suitable for the examination of unstained preparations, because the 

 light focussed by the condenser is so intense that it drowns unstained objects 

 and renders their outline very indistinct. 



4. The nosepiece. 



The nosepiece in most general use is constructed to carry three objectives 

 usually a No. 2, and a No. 8 or No. 9 dry, and a T \vth homogeneous immer- 

 sion lenses. Each objective is screwed into its proper place in the nosepiece, 

 which is marked for the purpose ; it is necessary that this be done in order 

 to get the centering true. By simply rotating the nosepiece, it is thus possible 

 without unscrewing them to use any of the objectives. 



5. Eyepieces. 



In the great majority of cases a low-power eyepiece should be used. A 

 high-power eyepiece only magnifies at the expense of brightness and sharp- 

 ness (p. 113). Eyepieces I. or II. are generally used, III. and IV. only when 

 delicate work requiring considerable magnification is in hand. 



6. Focussing. 



Focussing is done in two stages. The object is first brought approximately 

 into focus with the coarse adjustment, and then sharply focussed by means 

 of the fine adjustment. 



The focal length varies with the different objectives, being in inverse ratio 

 to the magnification. The approximate focus for each objective is soon 

 learnt with a little practice, so that the first stage of the process is quickly 

 done. 



The object having been brought more or less into focus with the aid of 

 the coarse adjustment, is exactly focussed by means of the fine adjustment 

 working on a micrometer screw. 



When high powers are used the objective will be close to the cover-glass, 

 and a rough movement downwards of the lens will most certainly break the 

 slide. [Microscopes are now made so that it is impossible to force the objec- 

 tives through the cover-glasses.] In any case, to avoid this possibility 

 proceed as follows : 



1. Before looking down the microscope, fix the eye on the preparation, 

 and lower the tube slowly with the coarse adjustment until the front lens 

 touches the cover-glass. 



2. Now look down the tube, and raise the coarse adjustment until the 

 preparation is approximately focussed. 



3. Then get the exact focus by gently rotating the fine adjustment. 



The fine adjustment should never be used for large alterations of focus ; it is a 

 very sensitive and delicate screw, acting on the microscope tube through a spiral 

 spring, which would soon be put out of gear if used for large excursions. 



While the fine adjustment is being used, the thumb and index finger of the 

 right hand should not be taken off the micrometer screw, but should con- 

 tinually move it backwards and forwards gently, until, without any effort 

 of accommodation, the different parts of the preparation are brought into 

 focus and seen in succession, and the shape of the object distinctly made out. 



While examining a preparation, the slide should be held between the 

 thumb and index finger of the left hand, and moved about on the stage, so 

 that the different parts can be brought within the field as required. [It is a 

 great advantage to have a " mechanical stage " fitted to the microscope ; 



