414 MANIPULATION AND PKESERVATION OF THE MICROSCOPE 



condensers by the method shown in fig. 347. But when the micro- 

 scope is of necessity used upright the rectangular prism or the plane 

 mirror must be used, fig. 34'8. 



The arrangement at fig. 356 is sometimes useful for photo- 

 micrography when it is otherwise impossible to illuminate the whole 

 field. But in ordinary cases it is better to contract the field than 

 use a bull's-eye, as it invariably impairs the definition. 



FIG. 356. Illumination for dark ground (with 

 stop beneath the condenser). 



FIG. 357. Same result with concave 



In regard to this last figure it will be understood that (as before) 

 E represents the edge of the flame, B the bull's-eye, M the mirror, S 

 the condenser under the stage, and F the plane of the obejct. 



The same result as the above may be obtained by the concave 

 mirror (as shown in fig. 357) instead of the bull's-eye. But this 

 is a very difficult arrangement, yielding the best results only with 

 great application and care. 



But the supreme folly of using a concave mirror and a bull's-eye 



FIG. 358. Absurdity of using a bull's-eye 

 and a concave mirror. 



FIG. 359. Absurdity of using a bull's- 

 eye with the edge of the lamp flame 

 not in its principal focus. 



is shown in fig. 358, where is the concave mirror and (as before) S 

 the sub-stage condenser ; this secures a result as will be seen by the 

 relation of the light to the condenser (S) which is as far from what 

 is sought and desirable as it can well be, while another lesson of 

 great importance may be learnt from fig. 359, which illustrates 

 the error of not having the edge of the flame E in the principal focus 

 of the bull's-eye B. The rays converge on the condenser S, so that 

 it will become in all probability impossible to focus it on the 



