DARK-GROUND ILLUMINATION 347 



of some of the organisms which he studied will give an idea of 

 the general utility of the method for this class of work : 



Flagellata : 



Monas, Bodo, Euglena. 

 Bacteria. 

 Chlorophycece : 



Chlamydomonas, Pandorina, zoospores of Ulothrix, 



Coleochaete. 

 Phazophyccce : 



Scytosiphon. 

 Bryophyta : 



Spermatozoids of Marchantia. 



The actual results obtained are hardly of general interest, 

 but from the point of view of this discussion the interest attaches 

 rather to the method employed. 



II. Study of the Plant Cell and the Protoplast 



The great utility of the method in studying the structure of 

 colloids had been demonstrated by Zsigmondy, and as it was 

 generally becoming realised that protoplasm was colloidal in 

 nature, a study of the plant cell by this method was likely to 

 throw further light on the actual state involved. 



Observations in this direction were first made by Dr. 

 N. Gaidukov, the appearances of certain objects being de- 

 scribed in the Berichte der deutschen botanischen Gesellschaft for 

 1906. 1 Most of the published work on the subject is due to 

 Gaidukov, and a full account of his researches will be found in 

 his work, Dunkelfeldbeleuchtung und Ultramikroskopie in der 

 Biologie und in der Medizin? 



In the practical application of the method the observer is 

 confronted at the outset with numerous experimental difficulties, 

 chiefly perhaps in the task of finding suitable material for in- 

 vestigation. For observation with a sub-stage condenser the 

 material must be mounted in water in the usual way, and for 

 good results must be only one cell in thickness, otherwise the 



1 Gaidukov, N., Berichte d. d. bot. Ges., 1906 ; Unters. mit Hilfe des Ultra- 

 mikroskopes, p. 107; Weitere Unters., etc., p. 155; Uber tilt. Eigenschaften der 

 Protoplasten, p. 192 ; Ult. Unters. der Stdrkekomer, etc., p. 581. 



2 Gaidukov, N., Dunkelfeldbeleuchtung und Ultramikroskopie in der Biologie 

 und in der Medizin. (Gustav Fischer, 1910, 8 marks.) 



