340 THE SPONGIDA OR PORIFERA. 



Then again, when it is required to study minute living aquatic 

 forms, there is considerable difficulty in keeping the slide on the 

 microscope stage sufficiently moist. There is a capital paper by 

 Messrs. Dallinger and Drysdale in the Monthly Microscopical 

 Jour7ial, March, 1874, accompanied by illustrations, showing their 

 " method of preventing the drop of fluid under examination from 

 evaporating, so as to admit of continuous examination of the 

 same forms with the highest powers." This plan is re-told, in an 

 exceedingly clear style, by Saville Kent, as follows : — " It con- 

 sists firstly of a plain glass stage, about the one-tenth of an inch 

 thick, fitted so as to slide on in place of the ordinary sliding-stage 

 of the microscope. From the left hand anterior border of this 

 stage a projecting arm is produced, which carries a socket for the 

 reception of a small glass reservoir, about one-and-a-half or two 

 inches deep. The glass stage being too thick to work through 

 with an achromatic condenser and high powers, a circular aperture 

 of sufficient size is cut through it, and a piece of thin glass 

 cemented on its upper surface. A piece of blotting-paper is now 

 cut coinciding in form with the glass stage, but slightly smaller, 

 and with a tongue-like projection that lies along the projecting 

 arm, and dips down into the glass reservoir. A circular aperture 

 of larger size than the covering-glass employed is cut out of the 

 centre of this paper, such aperture, where a ^-inch cover is made 

 use of, being preferentially the ii/i6thsof an inch. The foun- 

 dation of the moist chamber is now complete, and it only remains 

 to provide the bounding walls. This Messrs. Dallinger and 

 Drysdale accomplish by means of a piece of glass tubing, about 

 one-and-a-half inch in diameter, cut to three quarters of an inch in 

 length. Across one end of this tubing a thin sheet of caoutchouc 

 is next firmly stretched and securely tied, and a small hole per- 

 forated in its centre. The tubing with its free edge, which should 

 be carefully ground, is now placed concentrically upon the glass 

 stage, over the aperture in the blotting paper, and the object-glass 

 racked down upon the perforation in the caoutchouc. The 

 caoutchouc should be sufficiently thin to offer no impediment to 

 the action of the fine adjustment, while it at the same time 

 clasps the object-glass firmly round its central perforation and in 

 combination with the lowermost, or free edge, resting on the 



