ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 235 



on the wave-length is but slight. For a given opening, however, the 

 length of the parallelopiped is proportionally very considerable. 



The arrangement represented in fig. 33 is much more advantageous. 

 Here the rays undergo a triple internal total reflexion. The figure 

 shows the course of the peripheral rays of the free aperture from 

 A B. The two trapezoids are brought into juxtaposition, and a phase- 

 retardation between the two components occurs if the acute angle of the 

 trapezoid is 73° 48*6'. At the oblique path through the air-layer 

 between the trapezoids the components polarized parallel to and per- 

 pendicular to the incidence -plane are, however, in great measure 



Fig. 33. 



unequally weakened. Accordingly, all the emergent rays will be some- 

 what elliptically polarized if the angle made by the vibration-plane with 

 the edge A B be chosen equal to 45°. The author calculates that if 

 this angle be altered to one of 49° 2', both components will finally 

 emerge with uniform intensity, and will he circularly polarized. 



If the two trapezoids are cemented on to one another with Canada 

 balsam, then the unequal weakening of the two components still happens ; 

 but the light is circularly polarized if the angle between the polarization- 

 plane of the incident Ught and of the reflexion-plane is again 45°. 



Leitz Reflecting Condensers for Dark-ground Illumination and 

 Ultra-microscopic Observations.* — In a descriptive and illustrated 

 catalogue with the above title the firm of E. Leitz deal with (1) the 

 Concentric Reflecting Condenser, for obtaining dark-ground illumination ; 

 (2) the Ultra Condenser, for bringing into view ultra-microscopic particles. 

 These instruments have already been noticed in the Journal. f In the 

 introduction to the pamphlet it is pointed out that dark-ground illumina- 

 tion and ultra-microscopical illumination are not necessarily the same 

 thing, and that it is desirable that this distinction should be observed in 

 the nomenclature used by makers for naming apparatus of this kind. 

 Brightly illuminated objects can be seen more distinctly on a dark back- 

 ground than on one which is itself bright. The result is that objects 

 which were visible on a bright ground become much more distinct, and 

 other particles which could not be seen before will now come into 

 view. These particles may be dimensionally well within the resolving 

 power of the Microscope. So far the case is one merely of dark-ground 

 illumination. But, in addition, other particles may become visible de- 

 spite the fact that their dimensions are considerably smaller than the 

 wave-length by which they are seen. Such particles are beyond the 

 resolving power of the Microscope, and the case is then one of ultra- 



* Descriptive Catalogue with above title. E. Leitz, London (8 figs.). 

 t See this Journal, 1910, pp. 761-2 ; 1911, pp. 97, 100. 



