398 E. A. SCHAFER. 



In the first experiments the gas regulator, which in that 

 case was made to depend on the expansion of air, was placed 

 within the body of the stage, but under these conditions the 

 temperature was found to vary within slight limits, according 

 to the varying pressure of the gas supply, just as an air 

 thermometer varies with the barometric pressure ; there is, 

 besides, a disadvantage in having the regulator at a distance 

 from the source of heat. By employing the expansion of 

 mercury to cut off all the superfluous gas, and by placing the 

 regulator directly over the flame, the utmost constancy and 

 delicacy are attained. 



It is much to be regretted that, although we can learn the 

 exact temperature of the chamber, we have at present no 

 means of ascertaining how much that of the object under ex- 

 amination may differ from this ; nevertheless, it is certain 

 that the proximity of the objective of the microscope produces 

 a considerable amount of cooling. If it be desired to reduce 

 this by warming the objective, it is not difficult to introduce 

 by means of glass T-tubes a secondary circuit of india rubber, 

 the middle of which shall coil around the objective, whilst 

 the ends shall be connected, the one with the ascending or 

 inflow tube of the primary circuit, the other with the 

 descending or outflow tube. 



The reader will have noticed that the method by which 

 the circulation is maintained in the apparatus here described 

 is precisely the same as that emf)loyed in the hot-water 

 apparatus now so extensively used for warming houses and 

 conservatories ; moreover, the principle of gas regulation is 

 familiar to every laboratory student, and the screw regulator 

 below is a modification of a contrivance used in some forms 

 of barometer for altering the level of the mercury. Nothing, 

 therefore, is claimed on the score of novelty, at the same 

 time it is hoped that this adaptation of ordinary means to 

 microscopical ends may prove of some service to the 

 histologist. 



' The author reo:iets tliat the figures have been executed on too small a 

 scale. The foUowiiii^ are some of the actual measurements (in inches and 

 fractions of an inch) of the different parts of the apparatus : 



Sfa(/e. — 3| X 1^ X -j^ inch ; diameter of central chamber of stage f inch ; 

 diameter of inlet and outlet tubes y'g inch; diameter of tube for thermometer 

 * inch. 



Reservoir. — Height 2 inches ; outside diameter If inch ; diameter of 

 cavity for gas-reguhitor 1 inch; stand for reservoir (containing pin-hole gas 

 burner) about 2^ inches high. The stand may be of the same diameter as 

 the reservoir, but, for the sake of stability, should have a broad, heavy foot. 



Gas-regulator. — Bulb If X |^ inch ; lumen of tube about -^^ inch ; length 

 of tube 3 or 4 inches; thickness of side-screw about i inch, 



