344 SCIENCE PROGRESS 



and are only rendered visible by the scattering of light which 

 they bring about. 



This general method of illumination, which is aptly called 

 dark-ground illumination, has been applied to observation with 

 the microscope. For use with low powers of the microscope 

 only, the method has long been known, but it is since the 

 beginning of the present century that the great development of 

 the method for high-power work has taken place. 



In 1903, by the employment of the ultramicroscope, Siedentopf 

 and Zsigmondy showed the possibility of demonstrating the 

 presence of particles which were below the limits of microscopic 

 vision. For a short general discussion of the principles and 

 methods of ultramicroscopy reference may be made to the article 

 by H. Thirkill in this journal for 1909. 1 



As there is a growing confusion with regard to terminology, 

 a few words on the subject may not be out of place. The term 

 " ultramicroscope " is best confined to the form of apparatus with 

 unilateral illumination as originally devised by Siedentopf, 

 although on the continent there is a great tendency to extend 

 the term. Sub-stage condensers especially designed to give dark- 

 ground illumination should be called dark-ground illuminators, 

 although in many cases it is possible by their means to observe 

 particles which are below the limits of observation with the 

 microscope with direct illumination. The apparatus of Sieden- 

 topf and Zsigmondy is thus a special means of producing dark- 

 ground illumination applicable for ultramicroscopic observation ; 

 but dark-ground illumination does not necessarily imply ultra- 

 microscopic vision. 



So also the newer illuminators, the Cardioid condenser of 

 Zeiss 2 and the Ultracondenser of Leitz 3 are best regarded as 

 dark-ground illuminators, although their light-concentrating 

 power is greater than that of the ultramicroscope. 



Attention will now be turned to the special subject under 

 discussion, and an indication will first be given of how the 

 method is best applied in the observation of suitable plant 

 structures. 



For microscopic observation, botanical objects have generally 



1 Thirkill, H., " Ultramicroscopy and Ultramicroscopic Particles," Science 

 Progress, 1909, p. 55. 



2 v. Zeiss pamphlets : " Mikro 306," " Cardioid Ultramicroscope." 



3 v. Leitz pamphlet* 



