ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



365 



when R. Szigmoudy aud H. Siedentopf demonstrated their apparatus 

 for rendering ultramicroscopic particles visible, at the meeting of the 

 Xaturforscherversammlung, in Cassel, they showed a new way for in- 

 creasing the efficiency of the Microscope, and opened a new field for 

 scientific research. The firm of C. Eeichert, of Vienna, has given con- 

 siderable attention to the manufacture and design of apparatus of this 

 description ever since its introduction, and has endeavoured to simplify 

 the same and make it more accessible and covenient in use. Various 

 considerations and experiments have resulted in the new reflecting 

 condenser. 



This new method of rendering ultramicroscopic particles visible is 



-Obj.T 



-t— c 



;— C 



^— a 



- a 



Fig. 57. 



on the principle of dark-ground illumination, the light which illuminates 

 the object having a greater aperture than the cone of light entering the 

 objective which produces the image. This relation of the illuminating 

 to the image-forming rays is the reverse of Siedentopf's method as 

 practised at present. The first method has the advantage over the 

 second of utilising the source of light much better. A second advantage 

 consists in the fact that any dry objective can be used without any 

 additions or alterations (such as stops, grinding part of front lens away, 

 etc.) ; moreover the small particles are seen clearly without the disturb- 

 ing diffraction rings which surround the images obtained with the 

 Siedentopf apparatus. 



This new reflecting condenser or spot-lens consists of a plano-convex 

 lens from which the central portion of the curved surface is ground 



