140 THE AMERICAN MONTHLY [May 



lei, light falls on a concave mirror the light becomes con- 

 vergent, and the focus will depend upon the curve of the 

 mirror itself. If the condenser be retained for these low- 

 power objectives, the upper lens should be removed or the 

 field of illumination becomes inconveniently circumscrib- 

 ed ; but for the reasons just given the plane mirror must 

 always be used with the condenser. We have not infre- 

 quently seen workers, otherwise experienced, having no- 

 ticed the increase of light obtained by the use of the con- 

 cave mirror without the condenser, using it for the same 

 purpose with the condenser, but forgetting that by doing 

 so they are throwing upon the condenser convergent in- 

 stead of parallel light, and making it impossible to satis- 

 factorily focus the condenser itself. 



For objectives above § inch the use of the condenser is 

 advisable, and for the higher powers imperative, if the 

 full value of the objective is to be realized. In Continen- 

 tal microscopes, until reeently, a sub-stage condenser was 

 the exception rather than the rule. In many English labo- 

 ratories its use, therefore, is even yet scarcely understood, 

 because of the curious prejudice in favor of the Conti- 

 nental stand ; but though the Continental makers turn out 

 excellent objectives, they have only latterly begun to re- 

 alize that an objective of high aperture has really no ad- 

 vantage over one of low aperture unless used with a suit- 

 able condenser. 



Light. — Having so arranged lamp and mirror that the 

 object is properly illuminated, it will be necessary then to 

 focus the objective upon the slide, and, having done that, 

 to adjust the condenser up or down, until the image of the 

 edge of the lamp flame appears distinctly in the centre of 

 the field. The light is thus correctly adjusted, its rays 

 being brought to a focus upon the object, and so entering 

 the objective undisturbed. To a beginner, the partially- 

 illuminated field seems strange ; but he must remember 

 that in a critical image it is necessary to get the maximum 



