ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



289 



Prof. Ranvier * recommended the apparatus shown in fig. 48. A 

 brass plate b has a circular aperture of 2 cm. closed with a plate of glass, 



Fig. 48. 



to which is fixed a smaller glass disc a, so as to leave a circular groove c. 

 When the cover-glass is put on there is 0*1 mm. between it and the 

 upper surface of the disc. Two holes pierced through the brass plate 

 longitudinally admit and draw off the gas. 



M. A. Nachet improved on this by the gas chamber shown in figs. 49 

 and 50, which has the advantage that the glass on which the liquids to 



Fig. 40. 



Fig. 50. 



be examined are placed can be raised or lowered by a fine micrometric 

 screw let into the thickness of the metal plate. By this means the 

 thickness of the layer of liquid beneath the cover-glass can be increased 

 or diminished. 



The modified cells of Nachet (for allowing culture "systems to be 

 multiplied indefinitely) which we described at p. 708 of Vol. III. (1880) 

 are shown in fig. 51. These were specially intended for use with the 

 Chemical Microscope, where the objective is beneath the slide. A brass 

 plate is attached to the stage and holds, by clips, the glass slip to which 

 the gas chamber is attached. This consists of a glass ring and two 

 tubes in one piece. The bottom of the ring is closed by a piece of 

 cover-glass, and is cemented over an aperture in the slide. The body- 

 tube and objective of the Chemical Microscope, it will be remembered, 



* 'Traite technique d'Histologie,' 1875, pp. 44-5 (1 fig.). 



