ZOOLOGY A.ND BOTANY, MICROSCOPY, ETC. 



713 



small, that an ordinary slide cannot be used ; and a cover-glass must 

 be used instead. With still higber powers, particularly those with 

 correction, where the frame cannot be brought so near to the lenses, 

 the apparatus is unsuitable. This inconvenience might be avoided 

 by causing a larger, brightly illuminated diaphragm to cast an image 

 and from this to produce a second at c c ; the first image could then 

 be brought nearer to the object if desired, and the action would be the 

 same as if a real diaphragm were in the place of the image. The 

 author, in order to use the api^aratus for higher powei'S, also describes 

 a modification by which the frame is placed above the eye-piece, 

 where a second image of the diaphragm is formed ; but he adds that 

 " this arrangement also is capable of improvement," 



In observations with the apparatus, it was remarked that when 

 the field of view was obscured, there was greater penetration. With 

 a bright field, for instance, individual bacteria could only be seen 

 when exactly in the plane of the focus ; those in an inclined or 

 perpendicular plane were only seen as points. When, however, the 

 field was darkened by means of the frame, each individual could be 

 followed in its movements. 



Bausch and lomb Optical Company's "Universal Accessory." 

 — This (fig. 1.52^ is mainly intended as a remedy for the want of a 

 substage. It consists of a brass base-plate to be laid on the stage, 



Fig. 152. 



Fig. 153. 



having a central opening surrounded by a countersunk bed, which 



holds a polarizing prism shown in position in the figui'C. This can 



be rotated by the milled edge of its broad circular top. On removing 



the polariscopc a hemispherical lens can be dropped into the oj)ening 



in the plate, and serves as a condenser or, with a stop 



placed on it, as a paraboloid. An ingenious arrangement 



has been adojjted to enaV)le the lens to be retained in 



place. A disk of tliin glass of sliglitly larger diameter 



than the plane face of the loiis is cemented to it (fig. 153) 



so as to leave a projecting rim. This rim rests on the margin of the 



opening, and prevents the hemisphere passing through. 



Illumination.* — Mr. E. M. Nelson writes as follows: — The first 

 step in studying the principles of illumination for the Microscojio is 

 to grasp thoroughly the various effects produced by a bull's-eye. 



• En|?l. Mech., xl. (]88>) p. 08 (2 figs.); pp- 157-8 (.1 figs.); p. 20.T (0 figs.); 

 p. 282 (C fig8.). 



