HISTOLOGICAL METHODS. 147 



ture of the copper disc is indicated by a thermometer (/). The tempera- 

 ture which it shows, however, is only approximately that of the object. 

 This method of heating such a stage is convenient, because of the 

 rapidity with which it may be used. When, however, a constant 

 temperature is to be maintained for any considerable length of time, 

 the stage is best heated by a stream of hot water, as suggested by 

 Strieker and Sanderson (Fig. 49). Before passing to it, however, it 

 should be stated that the tubes a and a (Fig. 48) are inlet and outlet 

 tubes for the conveyance of gases through the cell. The wall of the 

 tube a is used as a heat-conductor merely for convenience. 



261. Strieker and Sanderson's Hot Stage and Gas 

 Chamber is the best for maintaining a constant tempera- 

 ture. It (Fig. 49) is a hollow flattened brass box through 

 which hot water flows by the 



tubes / and o. There is a 

 central cell similar to c in Fig. 

 48, with a thermometer (/) 

 coiled round it. Two tubes, 



a and b, for gases, COmmuni- FIG. 49. Strieker and Sanderson's 



cate with the cell. Recently 



Dr. Sanderson has adapted to this stage the wire-heating 

 apparatus shown in Fig. 48, so that the same stage may be 

 heated by the wire or by hot water a great convenience.* 



262. The simplest arrangement for supplying a stream 

 of hot water at a constant temperature is that (Fig. 50) de- 

 vised by Mr. Schaefer (Quarterly Microscop. Journ., vol. xiv. 

 p. 349). a represents a vertical section of the cell in the 

 stage, e e is an indiarubber tube for the circulation of 

 water through the stage from the reservoir B. The water 

 is heated by a gas-jet, the size of which is regulated by the 

 expansion of mercury in the apparatus A. When the stage 

 has attained the desired temperature, the mercury A is 

 elevated by the screw S until it touches the metal tube,/ 

 The gas can then only issue through the lateral slit in the 

 tube, so that the jet is reduced to a very small size. It 

 readily enlarges, however, the moment the mercury falls, f 



These forms of the hot stage are only adapted for keep- 



* This form of the stage is made by Hawksley, Oxford Street, 

 London. 



t This apparatus is made by Casella, 147 Holborn Bars, London. 



