DARK-GROUND ILLUMINATION 345 



to be mounted between an object slide and a cover slip, so that 

 the ultramicroscope of Siedentopf is quite unsuitable for ordin- 

 ary work ; moreover, the method gives apparently no better 

 results for this class of work, and is considerably more difficult 

 to use, than various types of dark-ground illuminators. Most of 

 the sub-stage immersion condensers give good results for such 

 work as the study of small transparent structures, or for the 

 observation of the intimate arrangement of the living cell. 

 Gaidukov 1 also used, with considerable advantage in the case of 

 thick objects, Siedentopf 's 2 method of stopping out the central 

 portion of the front lens of the objective ; but observation is 

 rather difficult with this apparatus. 



Dry objectives give on the whole the best results, but the 

 apochromatic series is greatly superior to ordinary objectives. 

 If homogeneous immersion is used, a suitable stop must be 

 introduced, when very good results are obtained. A. E. Conrady 

 has recently shown 3 that the maximum resolving power with 

 dark-ground illumination is obtained when the condenser has 

 not less than three times the N.A. of the objective. 



The centring of the sub-stage condenser is very important. 

 As a source of light a good Nernst lamp is sufficient for some 

 work on ciliation and the study of bacteria, but for the colloid 

 structure of the cell a small arc lamp is much more suitable and 

 shows particles which are missed with a weaker light. As a 

 condenser a spherical flask of water is very useful, and prevents 

 a large heating effect on the stage of the microscope. The 

 object slides — of selected thickness — and cover glasses should 

 be of good quality, specially cleaned, kept in alcohol, and rapidly 

 dried just before use. 



Work of a botanical nature which has been done by the 

 application of this method falls generally into two main 

 categories, which will be considered separately. The method 

 has greatly facilitated the observation of small, transparent 

 structures such as cilia, and of minute bacteria in the living state, 

 and as a development of its application to the study of colloids 

 it has been applied to the optical analysis of the living plant cell 

 and the protoplast. 



1 Gaidukov, v. infra. 



2 Siedentopf, v. Zeiss pamphlet No. 228. 



3 Conrady, A. E.,/our. Quekett Micro. Club, xi. 1912, pp. 475-80 ; v, abstract, 

 Jour. Roy. Micro. Soc, April 1913, p. 210. 



