GROWING- STAGE FOR CONTINUOUS WORK 



343 



The glass with the flap of linen in it is now filled with water, 

 and the linen is wetted and wrung so as not to drip, and the whole 

 is very soon, by capillary action, constantly and evenly wet. A 

 drop of the fluid to be examined must now be placed at k, fig. 293, 

 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 linen. 

 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 

 linen, 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 linen, 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 effect this is 

 simple. A piece of glass 

 tubing, about 1^ inch in dia- 

 meter, is cut to about J 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. 295 gives a 

 drawing of it ; a is the piece j *""" 

 of glass tubing, b is the F IG . 211 o. 



stretched elastic film, which 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 

 linen of the stage, the aperture c being over the centre of the cover- 

 ing glass. The object-glass is now racked down through the small 

 hole, c (fig. 295), and adjusted to focus. The caoutchouc should be 

 thin enough to afford 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 linen, so that little or no air is 

 admitted, while if the under edge of the chamber be carefully 

 ground it will suffer the stage, linen and all, to move under it when 

 the milled heads for working the mechanical stage are in action. 



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

 dicular section at fig. 296. The parts a, a in this figure represent 

 the glass stage corresponding to a, , fig. 293 ; b in both figures 

 stands for the round aperture in the thick glass ; b, in fig. 296, cor- 

 responds to the thin glass which covers this aperture, marked c, d, 

 e, f in fig. 293 ; but in the form of this device now used by the 

 Editor the thin glass floor is cemented to the bottom of the plate 

 glass, a, a, thus making a cell equal to the thickness of the whole 

 stage. The linen is marked in dotted lines in both figures : d, 

 fig, 296, represents the covering glass, /, in fig. 293 ; e, e, fig. 296, is 

 the piece of glass tubing shown in fig. 295; /,/, fig. 296, is the 



