THE COMPOUND MICROSCOPE. 



43 



light, excepting that required for the formation of the 

 image ; a further magnification of the image is effected 

 by the eye-lens e e, and precisely as if it were an 

 original object. 



The Stage. The stage of the 

 microscope should be perfectly flat 

 and rigid, without flexure, and as 

 thin as may be consistent with 

 these essential qualities. It should 

 rotate on its axis, as a revolving 

 stage possesses great advantages. 

 It enables the observer to keep a 

 diatom in view, while it is pre- 

 sented in succession to rays of 

 greater or less obliquity, and thus 

 a better insight is obtained into 

 structure. Supposing fig. 26 were 

 an object marked by longitudi- 

 nal strise, but too faint to be 

 seen by ordinary direct light, the 

 light most useful for bringing 

 these into view will be that pro- 

 ceeding obliquely in either of the 

 directions C and D ; whilst rays of 

 light falling upon it in the direc- 

 tions A and B would tend to ob- 

 scure the strias rather than disclose 

 them. If the markings, however, 

 are due to furrows or prominences 

 having one side inclined and the 

 other side abrupt, they will not be 

 brought into view indifferently by 

 light from C, or from D, but will be 

 seen best by that which produces 

 the strongest shadow ; hence, if 



there be a projecting ridge, with 

 an abrupt side looking towards c, 

 they will be best seen by light from D ; if, however, there 

 be a furrow with a steep bank on the side of c, they 

 will be seen by light from the same side. It not 

 unfrequently happens that longitudinal strise, or lines, 



