ACCESSORY APPARATUS. 125 



part of which should be almost flush with the general surface of the 

 plate), and the thin glass-cover being placed upon is, the drop spreads 

 itself out in a thin film, without finding its way into the deep furrow 

 around it; and thus it holds-on the covering-glass by capillary attraction, 

 while the furrow serves as an air-chamber. If the cover be cemented 

 down by a ring of gold-size or dammar, so that the evaporation of the 

 fluid is prevented, either Animal or Vegetable life may thus be main- 

 tained for some days, or, if the two should be balanced (as in an Aqua- 

 rium), for some weeks. 1 An improvement has been devised by Dr. Ed- 

 monds in the form of this Annular Cell; which he also makes to serve 

 as a ' gas-chamber ' for the introduction of gases or vapors into the Annu- 

 lar space. The central prominence is shaped as a truncated paraboloid;: 

 and while, by focussing in the object a 2-inch objective used as a conden- 

 ser, a bright field is obtained, this may be exchanged for a dark field by 

 putting the condenser out of focus (so that its light is thrown on the 

 sides of the paraboloid), and by gumming a black disk on the centre of 

 its under surface. A straight groove being cut in the slide, parallel to 

 its long side, and tangentially to the annular groove which it should 

 equal in depth, two fine glass tubes are cemented in it; one of them, 

 which is left projecting beyond the end of the slide, being connected 

 with a slender elastic tube through which gases or vapors may be pro- 

 jected into the annular space, while the other serves to convey them 

 away. 2 



124. Zoophyte Trough. For the examination of larger aquatic Ani- 

 mals or Plants under low or moderate powers, recourse may be advanta- 

 geously had either to the original Zoophyte-trough of Mr. Lister (which- 

 is still kept on sale by most Makers), or to a form lately devised by Mr. 

 Botterill, which has several 



advantages over the older one. Eio. 99.. 



This consists of two plates of 

 vulcanite, a back and a front, 

 shaped as in Fig. 99, connected 

 together by three brass screws; 

 these, being fixed in the faack 

 plates, pass through the front, 

 where their projecting ends are 

 furnished with small milled- 

 heads. Between these plates Botteriirs Zoophyte Trough, 



are two rectangular plates of 



glass, cut to such a length as to lie between the two side-screws of the 

 vulcanite plates, and having such a breadth that while their lower 

 edges rest on the bottom-screw, their upper are flush with the top of the 

 vulcanite disks. The glass plates are kept apart by a half -ring of vul- 

 canized india-rubber, of such a diameter as to lie just outside of the semi- 

 circular margin of the vulcanite plates; and they thus form the sides and 

 bottom of a trough, which is made water-tight by a moderate pressure 

 exerted by turning the milled-heads. The space between the two glass 

 plates may be varied by using half-rings of different thicknesses; whilst, 

 if it be desired to use a higher power than will work through ordinary 

 glass, a front plate of thin glass may be substituted. One great advan- 

 tage of this arrangement is the facility with which the pieces composing 



1 " Journ. Roy. Microsc. Society," Vol. ii. (1879), p. 55. 



2 Ibid., Vol. iii. (1880), p. 585. This Parabolized Gas-Slide is made by Messrs.. 

 Beck. 



