ITS MANAGEMENT. 163 



be unscrewed, or tampered with, at the risk of 

 damaging the cement which binds the lenses together. 

 If the objective be an immersion, carefully wipe off 

 the fluid from the front lens as soon as it is done with, 

 for even distilled water will leave a stain behind. 



When looking through the eye-piece be sure to 

 place the eye close to the lens, otherwise the whole 

 field will not be perfectly visible ; it should appear as 

 an equally well-illuminated circular disc. The position 

 of the observer should be easy and comfortable, and 

 the microscope inclined to an agreeable working angle 

 This will prevent fatigue and congestion of the eyes, 

 the first indication of which is small bodies moving 

 about or floating before them. If the eyelashes are 

 reflected from the eye-glass, the observer is looking 

 upon the eye-piece, and not through it. Eor the 

 examination of transparent mounted objects, it is 

 simply necessary to place them upon the stage of the 

 microscope, and throw light from the concave mirror 

 through them. The distance at which the mirror 

 should be set depends upon the source whence the 

 illumination is derived, and whether it be daylight or 

 lamp-light. The stem which carries the mirror is 

 generally so arranged as to be capable of elongation. 

 The working focal distance of the mirror is that which 

 brings the images of the window bars sharply out upon 

 the glass slip or object resting upon the stage. In 

 other words, the focus of the mirror is that which 

 brings parallel rays to a correct focus on the object- 

 glass. If employing artificial light, then the flame 

 of the lamp should be distinguishable ; a slight change 

 in the inclination of the mirror will be required to 

 throw the image of the lamp-flame out of the field. 



A good illumination having been obtained, the 

 diaphragm must be brought into use to regulate the 

 amount of light. The more transparent the object, 

 the less light will it require to display it properly. 

 Some microscopists carefully tone down the light, by 

 interposing a piece of monochromatic glass, or a fluid 

 medium, a weak solution of sulphate of copper, between 

 the light and the object. The best artificial source 



