98 Transactions of the 



covers.* The vessel with the flap of blotting-paper in it is now 

 filled with water, and a drop of water is placed on several parts of 

 the paper, and the whole is very soon by capillary action thoroughly 

 and evenly wet. A drop of the fluid to be examined must now be 

 placed at k, and the covering glass i must be laid on. It will be 

 seen that there is a broad clear space between the covering glass and 

 the bibulous jmper. We now want to form a chamber into which 

 the object-glass can be inserted, and which shall enclose a portion 

 of the constantly wet blotting-paper, and be to a very large extent 

 air-tight. The consequence will be, that the evaporation within 

 the chamber will be always greater in quantity from the blotting- 

 paper, on account of its continual renewal, than it can be from the 

 film of fluid. Indeed, the moisture in the chamber is so great 

 under favourable circumstances, that it rather increases than allows 

 a diminution of the film of fluid. The manner in which we efiect 

 this is simple. A piece of glass tubing, about 1\ inch in diameter, 

 is cut to about f of an inch in length. At one end of this a piece 

 of thin sheet caoutchouc is firmly stretched, and a small hole is 

 made in its centre. Fig. 3, PI. LIIL, gives a drawing of it ; 

 a is the piece of glass tubing ; h is the stretched elastic film, vrhicli 

 is securely tied on by means of a groove in the glass at d ; and c is 

 the aperture. The bottom edge e should be carefully ground. This 

 is laid in the position in which it is looked at in the drawing, on the 

 blotting-paper of the stage, the aperture c being over the centre of 

 the covering glass. The object-glass is now racked down through 

 the small hole e, Fig. 3, and adjusted to focus. The caoutchouc 

 should be thin enough to afi'ord no impediment to the action of the 

 fine adjustment ; when it will be seen that it clasps the object-glass 

 by its elasticity at the aperture ; and the gentle pressure forces the 

 under edge of the chamber upon the blotting-paper, so that little or 

 no air is admitted; while if the under edge of the chamber be 

 carefully ground it will sufier the stage, bibulous paper and all, to 

 move under it when the milled heads for working the mechanical 

 stage are m action. 



A drawing of the apparatus in working order is given in per- 

 pendicular section at Fig. 4, PI. LIII. The parts a, a, in this figure 

 represent the glass stage a, a, Fig. 1. 6 in both figures stands for 

 the round aperture in the thick glass, c in Fig. 4 corresponds to 

 the thin glass which covers this aperture marked c, d, e,f, in Fig. 1. 

 The blotting-paper is marked in dotted lines in both figures. 

 d, Fig. 4, represents the covering glass i in Fig. 1 ; e, e, Fig. 4, is 

 the piece of glass tubing a. Fig. 3 ; /, /, Fig. 4, is the stretched 

 caoutchouc seen at h in Fig. 3, with the object-glass g. Fig. 4, 

 penetrating and tightly filling up the aperture c in Fig. 3 ; thus 

 forming the moist chamber, ch, ch, by enclosing parts h, h, Fig. 4, 



* We find that the cover one quarter of an iucli and (he aperture in the 

 blotting-paper ^^ths workb well. 



