734 



SUMMAKY OF CURRENT RESEARCHES RELATING TO 



by cedar-wood oil. In consequence of the total reflexion at the cover- 

 glass, a very useful dark ground can be obtained with dry systems. 



The author's paraboloid condenser (fig. 118) is an improved form 

 of Wenham and Stephenson's, and can be fitted on to every Microscope 

 which possesses a condenser push-sleeve of ordinary width (36*8 mm.). 

 It is inserted in the position of the condenser sufficiently far to bring 

 its upper plane approximately to the level of the stage, and a cedar- 

 wood oil connexion, as far as possible bubble-free, is made between the 

 under side of the slide and the condenser. The thickness of the oil- 

 layer is 1*0 to 1'5 mm. The illuminating beams have a N.A. of 

 about 1 • 1 to 1 ■ 4 and are totally reflected at the upper plane of the 

 cover-glass in contact with air. The paraboloid has improved spherical 

 correction ; but its main advantage is that it reflects the rays instead 

 of refracting them. Dry systems of medium strength are used, Zeiss 

 DD with correction-collar being the most suitable. The best results 



Fig. 118. 



are obtained when this objective is screwed on to the tube with a small 

 centring arrangement ; the focus of the paraljoloid can then be made 

 to coincide with that of the objective. Strong compensation oculars 

 (No. 12 or 18) complete the optical equipment. Incandescent mantles, 

 Nernst-light, or, best of all, electric arc-light may be used. Sunlight, 

 or arc-light with this condenser, is sufficiently intensive for the instan- 

 taneous photography of living bacteria. The path of the rays in the 

 paraboloid is shown in the figure, those refracted in the object itself 

 being dotted. P is the plano-convex glass whose convex curve is an 

 accurate paraboloid of revolution. B is the central diaphragm, which 

 stops off rays of aperture ' to 1 • 1 ; it is covered with tin-foil to 

 prevent over-heating. is the object-slide, and in its upper surface is 

 the focus of the paraboloid. There are no diffraction circles apparent 

 when central illumination is used ; and, if the adjustment is carefully 

 performed, a living Spirochceta may be seen in five different typical 

 appearances. 



