768 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



and this will extend along the tube as the mercury retreats to the lower 

 reservoir. As the lamp is somewhat sensitive to pole connections, the 

 plug should be reversed should the surface of the mercury appear to 

 boil as it retreats along the tube. This procedure usually has to be 

 repeated two or three times before the lever E can be released. 



Fig. 167. 



New Reflecting Condenser.* — W. von Ignatowsky describes a new 

 reflecting* condenser which has been made at the works of E. Leitz. 

 The necessary data were supplied by the author, and the condenser 

 has been in use since October 1907. The fundamental principle of 

 the method of observation in a dark field is the modification of the 

 incident pencil in such a way as to establish a marked contrast be- 

 tween vividly illuminated particles, e.g. bacteria, and a dark ground. 

 As the details to be distinguished become finer, it is necessary to 

 increase the intensity of the illumination so as to enable the particles 

 to emit a sufficient amount of diffused light. Reflecting condensers and 



* Zeitschr. wiss. Mikrosk., xxv. (1907) pp. 64-7 (2 figs.). 



