ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



711 



appear somewhat darker than the other portions of the lens ; the 

 diflference, however, in the intensity of light is so slight, that it would 

 not generally be remarked. At the instant, however, that the dia- 

 phragm passes h and the lens becomes dark, only those rays remain 

 which in the figure are seen to pass below b, and g i will appear 



Fig. 149. 



Fig. 150. 



brightly illuminated on a dark ground. If g i is not in the lens but 

 in a medium before or behind it, as x y, the result is precisely similar. 

 The same effect is produced if g i has a lower refracting power than 

 the other part of the lens. 



Professor Topler recently drew Herr W. Seibert's attention to 

 the fact that the same principle 

 might be employed for Micro- 

 scopes, and the latter has accord- 

 ingly constructed an apparatus 

 which he says acts admirably with 

 low powers. The semicircular 

 diaphragm a, fig. 150 (the straight 

 edge perpendicular to the plane 

 of the paper), is so placed, that 

 its inverted image appears in h, 

 at which point a frame c c moves 

 in a lateral slit in the nose-piece 

 n n. This frame has at <Z a glass 

 plate, unpolished on the under 

 side, and at e a thin metal plate 

 with a bevelled edge. The space 

 between d and e is open, so that 

 in placing the frame in position, 

 as in the figure, all rays pro- 

 ceeding from the object pass 

 without hindrance. The screw g 

 holds the frame in position ; if it 

 be loosened, the latter can bo 

 moved by the hand. The final 

 adjustment is effected (after g has been screwed up) by the fine screw h. 

 Fig. 151 is a front view of the frame. In ordinary vision through 

 the Microscope the field is brightly illuminated by the rays which, 



3 A 2 



