ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



703 



required for the investigations of Siedentopf and Zsigmondy on ultra- 

 microscopical particles. Some of the results of these inquiries, more 

 especially in regard to gold ruby glass and bacteria, were described by 

 H. Siedentopf in a paper read before the Society.* It will be found, 

 on reference to the paper, that the principle of the experiment consists 

 (pp. 575, 576) "in illuminating only those particles which are to be 

 made visible, by focussing an arc light upon a small spectroscopic slit ; 

 the light from this slit being focussed by a condenser upon those par- 

 ticles which are to be made visible. The size of the slit can be precisely 

 controlled, and, with a knowledge of its width and of the condensing 

 system employed, the exact thickness of the layer of illuminated particles 

 can be regulated to a nicety." 



The axis of the Microscope must be at right angles to the plane of 



Fig. 107. 



the beam, and the general arrangement of the apparatus is shown in 

 fig. 107. On the stand «, an optical bench b, one metre long, is mounted. 

 The stand is 34 cm. longer than the bench ; the free part is unused, in 

 case of sunlight, but serves for an arc lamp in case of artificial light. 

 With sunlight a heliostat is indispensable. The figure shows at c a 

 self -regulating arc lamp mounted on the stand. The lamp is so arranged 

 that the axis of the narrow light beam emerging through the previously 

 erected diaphragm d is parallel to the optical bench. A small projection 

 objective / of 80 mm. focus, mounted on a rider, is arranged about 

 41 cm. from the beginning of the bench. The objective is surrounded 

 by a circular diaphragm disk of 15 cm. diameter to keep off side light, 

 and is chromatically and spherically corrected. 



The next piece of apparatus g is a precision slit-head, and is also on 

 a rider. It is moved along the optical bench until the projection objec- 



* Journal R.M.S., 1903, pp. 573-S. 



