ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 127 



one wMch he made in 1883. A similar condenser was supplied to a 

 Fellow of the Society in January of that year. As will be seen from 

 the figure, there is a cone A with convex under-surface ; the lens B is 

 however nearly in contact with the convex surface, whilst Dr. Wallich 

 separates them somewhat. 



Mr. Swift further says that finding the cone gave confused pencils 

 of light instead of convergent rays, he (in 1884) substituted for it an 

 achromatized paraboloid, with a flat top for immersion. This is 

 shown in fig. 18. A lens B is cemented to the lower surface of the 

 paraboloid A, with a second lens at C. It screws on the same form 

 of substage-fitting as the cone. 



In each form an annular diaphragm (C and D) slides inside the 

 adapter so as to be in contact with the lower lens. 



Wallich's Condenser.* — Dr. G. C Wallich more fully describes the 

 advantages of his condenser as follows : — 



" The principle I have kept in view in the production of the con- 

 denser is that for the illumination of transparent objects in the 

 Microscope, the conditions as regards light should as closely as pos- 

 sible be assimilated to those under which we are accustomed to look 

 at transparent objects with our unaided eyesight — that is to say, by an 

 ' all round ' light, in which, however, the rays proceeding from any 

 selected azimuth are partially, or, if need be, wholly, cut off. This 

 end is attained by a very simple and well-known mechanical contriv- 

 ance, inasmuch as the condenser itself (when fitted, as it can quite 

 easily be, to the substage of any Microscope) hardly ever requires to 

 be moved out of the axis of the instrument. Hence, it may be regarded 

 as supplying the very opposite mode of illumination to that usually 

 implied by the term ' oblique.' 



Again, instead of seeking to bring the whole of the utilized rays 

 to a focus in a point at the centre of the object under examination, or, 

 in other words, instead of securing only a single brilliantly illumi- 

 nated focal plane extending circumferentially and horizontally out- 

 wards from one focal point, it has been my aim to produce (both in 

 the monocular and binocular Microscope) a succession of brilliantly 

 and nearly equally illuminated horizontal planes extending to the 

 very margin of the field, and yet allowing the formation of just 

 sufi&cient shadow to secure the desired results. 



What these results are I will now briefly describe, premising, 

 however, that I expect no one to accept my statement without having 

 an opportunity of testing its accuracy by ocular demonstration. I 

 consider this proviso as all the more essential, inasmuch as, if my 

 statement bears the test, two or three important dogmas in the theo- 

 retical optics in relation to the Microscope will, undoubtedly, have to 

 undergo a certain amount of revision. 



In the first place, I claim that the condenser increases to a very 

 great extent the range of penetration at our command ; and that through 

 its means we are enabled to see a transparent object, or any number of 

 transparent objects, at a single focusing, very nearly as distinctly as 



* Eug]. Mech., xl. (1884) p. 320. 



