614 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



burner is so arranged that increase of the gas snpply, so long as the 

 index is on the first half of the scale, is comparatively slight ; but on 

 the second half each scale division implies a greater increase of gas 

 supply. In heating to high temperatures, gas is admitted through the 

 other nozzle s, air being passed through CI, and the small cap r is 

 screwed on to the point of the burner. The two scales serve, first, to 

 afford an accurate mark for the adjustment of the burner ; secondly, to 

 provide a method of applying a systematic course of heating. As the 

 heating of the preparation, when the flame is very small and therefore 



Fig. 91. 



comparatively far from the preparation, is not lateral to the axis, the 

 heat-guide e is inserted from above through the stage aperture of the 

 Microscope. This guide is a conical brass tube with an external asbestos 

 coat. 



The attainment of a temperature constant for a long time is assisted 

 by a downward cooHng, which, moreover, steadies the under heating. 

 Lehmann's air-cooHng arrangement (fig. 91) used for the purpose is made 

 of a brass rod of rectangular cross section perforated with an air-canal. 

 This canal has on its upper side two openings on which, by means of 

 air-tight ball-joints, two adjustable air-tubes are placed. In the long 

 axis of the brass rod there is, on one side, a nozzle for the air-supply and 

 on the other a regulating screw for the strength of the air-current. 

 The brass rod is fitted on both sides with projecting arms so arranged 

 that the whole can be laid around the flange at the foot of the upper 

 part of the Microscope used, and can be tightened by a screw. The 

 "arms vary in form according to the flange, and are so adjusted that their 

 maximum effect is exerted on opposite sides of the field of view. The 



