Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



OCTOBER, 1913. 



DARK-GROUND ILLUMINATION AND ULTRA- 

 MICROSCOPIC METHODS IN BOTANY. 



By S. REGINALD PRICE, B.A. (Cantab.) 



During the last few years, and more especially 

 since the introduction of the ultra-microscope in the 

 early part of the century by Siedentopf and 

 Zsigmondy, the application of methods of dark- 

 ground illumination to high-power work has 

 received much attention, while many interesting and 

 important observations have been made by the 

 use of the methods. The work which has been 

 done in botanical research is probably compara- 

 tively little known, but nevertheless the method 

 seems likely already to become an important one in 

 studying various aspects of the plant cell and also 

 in other directions indicated below. The results, 

 moreover, are of general interest to the biologist as 

 well as to the specialised plant physiologist, and, in 

 view of this, a short account of some of the observa- 

 tions already made may be of sufficient interest 

 to justify this article. 



Only a very brief account of methods can be given 

 here, and at the outset it must be emphasised that 

 a distinction must be made between methods of 

 dark-ground illumination and the method of the 

 ultra-microscope. The latter term is best confined 

 to the method devised and used by Siedentopf and 

 Zsigmondy — the illumination of the object (coloured 

 glass, colloidal solutions, and so on) by a lateral 



beam of light, perpendicular to the optical axis of 

 the microscope. 



Dark-ground illuminators are sometimes also 

 called ultra-microscopes, but this terminology should 

 not be used, although it is true that at times they 

 can render visible, particles which are below the 

 limits of visibility in direct illumination. 



An apparatus for demonstrating ultra-microscopic 

 particles is always a dark-ground illuminator, but 

 the converse does not necessarily hold. Certain 

 forms of sub-stage illuminators are now on the 

 market, which give a very intense concentration of 

 the light in a small area, and these are chiefly used 

 for studying ultra-microscopic particles. Such are the 

 Cardioid condenser of Zeiss and the Ultra-condenser 

 of Leitz. Possibly the term " ultra-microscope " 

 should not be applied to these, if it be desired to 

 keep a special name for Siedentopf and Zsigmondy's 

 apparatus, but Leitz's term " ultra-condenser " would 

 be suitable. 



It is important to notice that most of the high- 

 power dark-ground illuminators, do render visible 

 ultra-microscopic particles under certain conditions. 



For further particulars the pamphlets of Leitz 

 and Zeiss should be consulted, as well as works on 

 the microscope, which, however, will not include 



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