344 ACCESSORY APPARATUS 



stretched caoutchouc seen at b in fig. 295, with the object-glass </, 

 penetrating and tightly filling up the aperture c in the figure, thus 

 forming the moist chamber, cA, ch, by enclosing parts A, A, fig. 296, 

 of the linen, which from the glass vessel to the left of the stage is 

 by capillarity always renewing its moisture ; and with 6, fig. 296, 

 sunk as a cell, by the attachment of the thin glass floor to the under 

 side of the stage, as described above, this annular flap of linen over- 

 hangs, but does not lie upon, the floor on which the drop of fluid 

 with its living inhabitants is placed. This is a great security 

 against accidental flooding. 



It will be seen that the microscope must be vertical ; but there 

 is no inconvenience arising from this if it be placed on a sufficiently 

 low support, and it will be found in practice that it may be worked 

 for a long time without any other change in the arrangement than 

 the screwing up or down of the fine adjustment. The difficulties in 

 working are few, and can be best discovered and overcome in 

 practice. 



Dr. Dallinger's Thermo-static Stage for Continuous Observations 

 at High Temperatures. It frequently happens that, either for the pur- 



FIG. 297. 



pose of experiment or the study of special organisms, the student 

 needs a similar continuous stage to the above, but one in which 

 varying temperatures may be obtained and kept at any point static 

 at the will of the operator. This is very satisfactorily accomplished 

 by the following device : The stage was made as described above, 

 but it was made hollow and water-tight. The whole stage is seen 

 in perspective in fig. 298. At A, a b are two grooved pieces of solid 

 metal which permit the stage to slide on to the stage of an ordinary 

 microscope, and partake of the mechanical movements effected by 

 the milled heads ; B is a vessel for water with a thermometer a 

 of sufficient delicacy for indicating the temperature ; b is a mer- 

 curial regulator, carefully made, but of the usual pattern ; c brings 

 the gas from the main ; d conveys as much of the gas as is allowed 

 to escape from between the top of the mercury and the bottom 

 of the gas delivery tube to the burner e. The regulation of this 

 apparatus so as to obtain a static temperature, as is well known, is 

 a matter of detail depending chiefly on the careful use of the mer- 

 curial screw-plug f and the height and intensity of the burner e. 

 A temperature quite as accurate as is needed for the purpose 

 required can be obtained. 



