100 SUMMARY OF CURRENT RESEARCHES RELATING TO 



plane-parallel piece; it has only the significance of au intermediate 

 piece, and serves tn approximate the upper Burface of the condenser to 

 the object-carrier. This peculiarity also makes it less sensitive to 

 injury in the use of high temperatures which might affect the cement. 

 Both mirrors are silvered so that only those rays emerge which are 

 required for dark-ground illumination; thus the concave mirror func- 

 tions also somewhat as a diaphragm stop. 



Fig. 9. 



Simple Method of Making Drawings for Projection Purposes.* 

 M. Ponzo pours filtered gelatin over glass plates ; old cleaned-up photo- 

 graphic plates do very well. When dry, these plates may be drawn on 

 with ink, Indian ink, anilin colours, with brush or pencil. Mistakes 

 may be remedied by scratching off the gelatin and refilling the erased 

 spaces with fresh gelatin. 



Electric Heating Apparatus for Microscopical Observations.f — 

 Figs. 10, 11, 12 show that the main idea in F. Jentsch's heating appa- 

 ratus is a rectangular or circular brass box. This contains the heating 

 apparatus, well isolated externally by means of asbestos : the box is 

 fastened by two screws on to a slate slab 6 or 7 mm. thick, and is 

 placed directly on the object-stage. The heating chamber is, in 

 figs. 10 and 11, a small hollow metal box of good heat-conductivity in 

 order to warm up the preparation uniformly, and is especially capable 

 of resisting loss of heat by the observation-hole. In pattern No. 1 

 (fig. 10) the stove has a certain slowness, so that heating up, and 

 cooling down, require some minutes (perhaps one minute for 250° C). 

 If the observation-hole is covered with a cover-glass a preparation may 

 be kept for a whole day at a constant temperature. This effect is 

 essentially due to a special spirally shaped coiling of the heating re- 

 sistance wire, which gives within wide limits a temperature gradient 

 proportional to the time. The highest temperature attainable depends 



* Zeitschr. Biol. Technik u. Methodik, ii. (1910) p. 46. 



f Zeitschr. wiss. Mikrosk., xxvii. (1910) pp. 259-64 (5 figs.). 



