ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



'09 



{2) For Ultra-microscopical Bacteria between Object-carrier and 

 Cover-glass. — The principle involved was described by Siedentopf in the 

 above-mentioned paper,* as follows : " In the arrangement for this 

 purpose the axis of the illuminating cone of light, and that of the rays 

 diffracted by the object, are in a straight line, and not at right angles 

 to each other, as in the other methods. Preparations of bacteria can 

 therefore be mounted in the usual way." The general arrangement of 

 apparatus is shown in fig. 115 ; and, on comparing it with fig. 107, it 

 will be noticed that the base-plate, the optical bench, the heliostat (or 

 arc lamp), and the projection objective have been retained. The 

 objective of the Microscope is illuminated by the rear focus of its Abbe 

 condenser, which itself receives the direct beam from the arc lamp 

 through the diaphragm d. The observation Microscope is arranged at 

 the end of the optical bench, and is secured on the stand Jc by means 



A. ».w. hunger; JENA 



Fig. 115. 



of clamps. It is set horizontally, so that its axis is parallel to that of 

 the light cone : the object-stage is therefore vertical. 



The illuminating apparatus consists of an exchange condenser 

 (fig. 116) which permits of an easy alteration from ordinary illumina- 

 tion to dark-ground illumination. It includes the push-tube a, the 

 three-lens condenser b, the special objective for dark-field illumina- 

 tion c, and the centring apparatus d. The tube a slides directly into 

 the sliding collar of the illuminating apparatus, and, when fully pushed 

 in, it engages with the clamp h of the condenser b. The iris diaphragm 

 with ground-glass disk is then inserted from the side, and the illumina- 

 tion now takes place in the ordinary manner (fig. 117). The handle m 

 serves for lateral movement of the iris. The objective used is the 



* Journal R.M.S., 1903, j>p. 577-8. 



Dec. 21st, 190.1, 



:) c 



