ZOOLOGY AND BOTANY, MICKOSCOPYj ETC. 



117 



aperture 40 mm. wide, which can be wholly or partially covered by two 

 sliding plates. 



A special Briicke magnifier (with variable power) screws on the 

 arm M. The arm has also a spring-tube into which a smaller mirror 



Fig. 13. 



Fig. 14. 



can be inserted. This is for illuminating opaque objects, and receives 

 its light from the larger mirror below. The focus of both mirrors is 

 so regulated that with high powers 

 the theoretically possible maxi- 

 mum of illumination can always 

 be nearly attained. For very 

 weak illumination there is on one 

 side a plate of opal glass. " The 

 mirror has the great advantage 

 over ordinary illuminating lenses 

 that the field of view is always 

 somewhat faintly and evenly illu- 

 minated, which extraordinarily facilitates the visibility of many natural 

 objects which have not sharp outlines." The upper mirror can be placed 

 in any position with regard to the axis of the lower, and can besides, 

 for special objects, be put in the spring-tube of the lower mirror. 



