346 ACCESSORY APPARATUS 



aperture, thus forcing the lower ground surface of the cylinder upon 

 the linen, and making the spare within the closed cylinder practi- 

 cally air-tight, but still admitting of capillary action in the linen. 

 Thus the enclosed air becomes saturated. 



l!y complete circulation the water in the vessel e (A) is but 

 slightly below that within the jacket of the stage, and thus the 

 vapour as well as the stage is near the same thermal point. 



For the admission of illumination and for allowing the use of 

 various illuminating apparatus, a large bevelled aperture e (C) is 

 made between the lower and upper plates of the stage jacket, which 

 is found to supply all the accommodation needed. 



There are many other forms of hot stage having various special 

 purposes, and some of general application ; a good account of these 

 will be found in the ' Journal Roy. Micro. Soc.' vol. vii. ser. ii. 

 pp. 299-316 and in subsequent volumes. 



The Live-box and Compressors. What is now so well known 

 even to the tyro as the ' live-box ' was originally devised by Tully, 

 and it was afterwards improved by Varley, who, in the place of a 

 level disc of glass for the floor, as well as the top of the ' box,' 

 bevelled a piece of thick glass and burnished it into the top of the 

 tube, where it formed the floor of this ' animalcule cage ; ' this 

 prevented the draining off of the water at the edge by capillary 



all rad ion. lint in that form a condenser cannot be used successfully 

 with it. and therefore a dark ground cannot be employed. But as it 

 is Rotifera and Infusoria generally that constitute the raison d'etre 

 for this piece of apparatus, and as a dark ground gives results of 

 high value- to say nothing of their beauty with these forms, it 

 lo>t much of its value. 



.Mi*. Rousselet has overcome these difficulties by a device which 

 is shown in fig. 299. 



In this the glass plate bevelled for the floor is somewhat reduced 

 in diameter, but the outer ring is enlarged sufficiently to allow any 

 liigli power to focus to the very edge of this glass floor. All object 

 lying anywhere over the floor can be reached bv the condenser from 

 below, and by both high and low powers from above, and when well 

 made it acts admirably as a compressor. A drop of water SO small that 

 a rotifer may be unable to swim out of the field of view of a |-inch 

 object i\c can lie readily arranged with it; and a little practice 

 enables the operator to employ it for many useful purposes in the 

 >t udv of pond life.' 



The compressor or compressorium is a more elaborate device, 

 someuhat of I he xmic kind, but arranged to give the operator 

 more accurate control over the amount of pressure to which the 

 object is subjected. .Mr. l!oii>selet has constructed one of very 



