178 MANAGEMENT OF THE MICROSCOPE. 



when illuminated from the opposite side. Hence it is always 

 desirable to try the eflect of changing the position of the object; 

 which, if it be "mounted," may be first shifted by merely re- 

 versing the place of the two ends of the slide, and then, if this 

 be not satisfactory, may be more completely as well as more 

 gradually altered, by making the object-platform itself revolve, 

 where the stage is fitted with such a movement: if, however, the 

 object be not mounted, but be simply resting on the stage-plate, it 

 may be readily shifted by hand. With regard to the obliquity 

 of the illuminating rays, it is well to remark, that if the object 

 be "mounted" under a glass cover, and the incident rays fall at 

 too great an angle with the perpendicular, a large proportion of 

 them will be reflected, and the brilliancy of the object will be 

 greatly impaired. 



93. The same general arrangement must be made, when 

 Artificial light is used for the illumination of opaque objects ; 

 the lamp being placed in such a position in regard to the stage, 

 that its rays may fall in the direction indicated in Fig. 55 ; and 

 these rays being collected and concentrated by the condenser, as 

 already directed. As the rays proceeding from a lamp within a 

 short distance are already diverging, they will not be brought by 

 the condenser to such speedy convei'gence, as are the parallel 

 rays of daylight; and it must, therefore, be further removed 

 from the object, to produce the same eflect. By modifying the 

 distance of the condenser from the lamp and from the object 

 respectively, the cone of rays may be brought nearly to a focus, 

 or it may be spread almost equally over a large surface, as may 

 be desired. In the illumination of opaque objects, the inferiority 

 of artificial to solar light is not so perceptible as in the case of 

 transparent objects; and the former has the advantage of being 

 more easily concentrated to the precise degree, and of being 

 more readily made to fall in the precise direction, that may be 

 found most advantageous. Moreover, the contrast of light 

 and shadow will be more strongly marked, when no light falls 

 upon the object except that proceeding from the lamp used for 

 its illumination, than it can be when the shadows are partially 

 lightened by the rays which fall upon the object from every 

 quarter, as must be the case if it be viewed by daylight. If the 

 ordiuaiy condensing lens do not aftbrd a suflicieut illumination, 

 the large " bull's-eye" condenser (§ 64) may be employed; its 

 convex side being turned towards the lamp, when it is desired to 

 bring its rays into the most complete convergence. And, if a 

 still more concentrated light be required for the illumination of 

 a small object under a high power, the small condenser may be 

 so placed as to receive the cone where it is reduced to its own 

 size ; since, by its means, the rays may be brought to a more 

 exact convergence than they can be by the bull's-eye alone. In 

 this manner, very minute bodies may be viewed as opaque objects 

 under a tolerably high magnifying power ; provided that the 



