ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



239 



with the focus C, and truncated to such an extent as to stop all light 

 of less obliquity than 1*0 N.A. from reaching the object. Therefore 

 when dry lenses, or oil-immersion lenses, with no greater angle than 

 1*0 N.A. are used, no direct light enters the Microscope, but the 

 objects are illuminated by an annular ring of very oblique light, and 

 are seen due to the light which they reflect. By moving the paraboloid 

 B up or down by means of the lower milled ring which rotates the 

 sleeve in which it is held, the lens A being retained in contact with the 

 slide, the light is accurately focused and the maximum brilliancy 

 obtained. Various forms of bacteria, viewed by this method, show 

 different structure, and it would appear to be a hopeful method of 

 obtaining an increased power of examining living micro-organisms. A 

 powerful light is essential. An incandescent gas lamp, with a bullseye 



Fig. 49. 



to project a parallel beam upon the mirror of the Microscope, gives good 

 results. The Nernst electric lamp forms an excellent light for this 

 purpose. But whatever light is used it should be parallelised by means 

 of a bullseye or aplanatic condenser. Fig. 49 shows the Nernst lamp 

 on stand complete with an aplanatic Herschel condenser. 



New Microscope Lamp.* — C. Troester has devised a lamp by which 

 light is transmitted from its source to the Microscope through a straight, 

 internally-polished tube (fig. 50). The source of light is an incandescent 

 burner, with a metal chimney having an opening in front. The tube is 

 so fitted that it can revolve in a vertical plane, and about a point in the 

 centre of the incandescent body. The Microscope mirror is placed close 

 to the end of the tube and arranged to catch the central beams. A 

 convex lens is inserted at the lamp end, and a blue glass disk at the 

 Microscope end. The light obtained is said to be more powerful than 



* Centralbl. Bakt., lte Abt. Orig., xlv. (1907) pp. 574-5 (1 fig.). 



