776 EREOES OF OBSERVATION. 



not so liable to injure the eye by overstraining it. The management 

 of light, either natural or artificial, is of the greatest importance in 

 microscopic observations. This may ie regulated by altering the position 

 of the mirror under the stage ; the proper adjustment of -which wOI soon 

 be acquired by a little practice and observation. In adjusting the 

 microscope for use, &st place it in its proper position, and screw or 

 slide on a low-powered object-glass, then look through the tube, and 

 incline the mirror towards the light, moving it about until a clear hright 

 light is seen. The object may then be placed upon the stage and the 

 focus adjusted by the rack movement. In examining any fresh object 

 the lowest magnifying power should be first used, as a larger portion 

 of it can be thus viewed at once, and a better general idea of its form, 

 colour, etc., obtained. Afterward the higher powers may be employed, 

 in order to reveal its minute structure. 



In viewing very delicate transparent objects, as fossil infusoria, 

 thin vegetable and animal tissues, blood and milk globules, etc., a 

 good clear light should be used, but the mirror should be inclined on 

 one side more than usual, that the object may appear less brightly illumi- 

 nated. This is what is termed " oblique illumination " — ^the rays of 

 light being reflected from the mirror, through the object, in an obliqw 

 direction, by which many delicate markings may be observed on some 

 objects which could not be distinguished before, and the outline also 

 rendered more distinct. 



In examining opaque objects, a low magnifying power should be 

 used, and the light thrown upon the object by means of the " Con- 

 denser," which should be placed within two inches of it, and so 

 arranged that a small circle of bright light may be seen upon the spot 

 to be examined. When viewing objects in a drop of water, or 

 examining a drop of any other liquid, a slip of thin glass should 

 always be laid over it ; otherwise the liquid wiU evaporate, and con- 

 densing on the object-glass, wiU render it dim. 



SouKCES OF Eeroes OP OBSERVATION. — Extraneous or accidental 

 objects may be present, and may be derived from various sources. 

 Thus, water too long used may bring before the eye both plants and 

 animals of the lowest forms, which otherwise would not have been 

 present. Fresh water is absolutely necessary. Particles of dust, or 

 fibres from the cloths used in cleaning the glasses, may also add to 

 the confusion. These consist, generally, of fibres of paper, linen, 

 woollen, cotton, or silk fabrics, or minute hairs from the brushes used 

 in manipulation. Air-bubbles are almost invariably a source of con- 

 fusion to the microscopic observer in his first attempts ; but once seen 

 and studied, they no longer distract the attention, and the microscopist 

 soon gets into the habit of disregarding their presence. When seen 

 by transmitted light they generally appear in the form of circles of 

 larger or smaller diameter, with a dark rim surrounding them ; while 



