MANIPULATION OF THE MICROSCOPE. 25 



oily, must never be allowed to touch the lens. Well- 

 defined spots seen on looking through the microscope at 

 a clear field are due to specks of dust upon the eyepiece, 

 as may be proved by rotating that system of lenses. Not 

 infrequently such specks are formed on the upper sur- 

 face of the lower or field lens of the ocular, which must 

 be unscrewed in order to remove them. Diffuse cloudi- 

 ness is generally caused by dirt or moisture on the object- 

 ive; and this may be removed by breathing on the glass 

 and wiping it with lens-paper. Sometimes for stubborn 

 smears of dirt or grease, 95% alcohol or xylol may be 

 used, but in the latter case the solvent must be sparingly 

 applied and proonptly removed in order that it may not 

 affect the setting of the lenses. 



2. Lighting. — ^After the objective has been screwed in 

 at the bottom of the tube, as shown in Fig. 18, and the 

 eyepiece has been inserted at the upper end of the draw- 

 tube, the mirror should be so adjusted as to give the best 

 available illumination. Direct sunlight must of course 

 be avoided, but a clear area of sky or a white cloud fur- 

 nishes an ideal source of light; and sunlight reflected 

 from a white wall or transmitted directly through a cur- 

 tain may be used to advantage. For artificial illumina- 

 tion electric bulbs with ground-glass shades are suitable, 

 or the narrow edge of the flame from a fiat-wicked oil- 

 lamp may be substituted. A gas-flame, unless some 

 incandescent mantle be attached to the burner, is not 

 satisfactory on account of its unsteadiness. 



The flat surface of the mirror yields sufficient light 

 under ordinary circumstances, but the concave surface 



