4^88 APPENDIX. 



D. MICROSCOPE AND MICROSCOPIC 

 EXAMINATION. 



i. In using the microscope the first thing to be considered 

 is the illumination. A position facing a window should 

 be selected, and if the window be a north one there will 

 be an advantage in the absence of direct sunlight, which 

 should never be employed for microscopic work. The 

 light from a south window is however equally good, if a 

 white blind be used to exclude bright sunshine. 



The mirror must be so placed that the field of the 

 microscope appears quite bright. The admission of 

 light from the mirror to the object is regulated by the 

 diaphragms. The size of the opening to be used should 

 depend on the magnifying power. With a low power a 

 large opening is necessary, or the whole field will not be 

 illuminated ; with a high power a smaller opening gives 

 a better definition, though it diminishes the intensity of 

 the illumination. Most modern microscopes are pro- 

 vided with a double mirror : the flat one should be used 

 low down in illuminating with a low power ; with a high 

 power on the other hand, the concave one should be 

 used obliquely and high up. 



2. The microscope has two adjustments of focus. The 

 coarse adjustment is effected by sliding the body of the 

 microscope up and down in its tube, or in the larger in- 

 struments by a rack and pinion movement. The fine 

 adjustment is worked by a screw. In all observations 

 the low power should be used first, and then the high, if 

 necessary. The focal distance of the low power is of 

 course relatively long, usually either an inch or half an 

 inch. Hence there is no difficulty in focussing and no 

 danger of crushing the object. With the high power, 

 which may have a focal distance of } in., J- in., or 

 less, great care is necessary, lest the objective should 

 touch the cover-slip. It is best to slide the body of the 



