346 SCIENCE PROGRESS 



I. Study of Minute Organisms and Ciliation 



In 1904 Rahlmann 1 showed that the method of Siedentopf 

 and Zsigmondy could be used with advantage for the study 

 of bacteria in the unstained condition. Even when of com- 

 paratively large dimensions these are difficult to observe in 

 direct illumination, but by the dark-ground method diffraction 

 images of even the very minute ones appear as bright patches 

 of light against a dark background. 



Cotton and Mouton 2 showed that observation of bacteria was 

 also possible with their special total reflection apparatus, and it 

 has since been shown that the sub-stage dark-ground illuminator 

 is in general very suitable for observations upon living bacteria. 



It is obvious from the theory of the method that true images 

 of the organisms are never obtained, but that generally a very 

 good idea of the actual form is given, since the diffraction images 

 are produced by objects whose dimensions are within the limits 

 of microscopic vision. 



A considerable number of observations have been made on 

 the flagella of living bacteria. As the observations are mostly 

 of interest to the specialist they will not be discussed further 

 here. For a list of some of the more important papers which 

 have appeared among a large literature, reference may be made 

 to the work of Dr. Gaidukov. 3 



The method also provided a means for study of the moving 

 cilia of motile algae, of zoospores, and so on. These extremely 

 fine and transparent structures when illuminated by this method 

 appear as bright moving lines against a dark background. As 

 is well known, it is much easier to see a bright line on a dark 

 ground than a dark line of the same width on a bright ground, 

 so that if this were the only effect the visibility of these cilia 

 would be greatly increased. As they are by no means black 

 lines when viewed in direct illumination, the contrast obtained 

 by the two methods of illumination is even more pronounced. 

 V. Ulelah 4 has recently published a series of researches on the 

 movements of cilia of various organisms, the observations being 

 performed by the aid of a Zeiss paraboloid. The following list 



1 Rahlmann, E., Munch. Med. Wochenschr. Nr. 2, 7S. 1904. 



* Cotton and Mouton, Les Ultramicroscopes, etc. Masson, Paris, 1906. 

 3 V. infra. 



* Ulelah, V., Biol. Centralblatl., 191 1. 



