742 



SUMMARY OF CURRENT RESKA.RCHKS RELATING TO 



set up on any table, but a special stand F has been designed for the 

 purpose (figs/l22 and 125). This stand has the advantage that, after the 

 removal of the drawing-board Z, a more highly magnified drawing may 

 be made on the lower table-top. Also that, owing to its greater height, 

 it is better adapted for projection on the screen. 



(5) Microscopical Optics and Manipulation. 



Polarisation of Refraction and Propagation of Light in a Medium 

 Non-homogeneous.* — C. Fabry discusses the observations of Salet, who 

 failed to detect evidence of polarisation in the light emitted from the 

 solar protuberance, as reported by certain astronomers. The passage of 

 a ray refracted through a series of media of progressive indices of refrac- 

 tion is evidently connected with the reflections which would take place 

 at the successive surfaces and which might result in some polarisation of 

 the ray. The author's experiments tend to support Salet's observations. 



Hartl, H.— Ein Modell zur Erlauterung der Zerlegung eines linear polarisierten 

 Lichtstrahls bei der Doppelbrechung. Zcit.f. Unterricht., xix. (1906) p. 175. 



KOEBBEB, F.— Ein Freihandversuch zur Bestimmung der Brechungsexponenten 

 des Glases. Tom. cit., p. 167. 



C6) Miscellaneous. 



Brownian Movement in Gases : its Visibility through an 

 Ordinary Microscope.f — F. Ehrenhaft has observed the above pheno- 

 menon by the aid of an ultra-Microscope ; but H. Molisch has found 

 that in many cases the use of an ordinary Microscope with weak objec- 

 tives suffices to render the Brownian movement visible, even with 



Fig. 126. 



ordinary illumination. A glass ring, about 12 mm. inner diameter and 

 3-5 mm. high, is cemented on to a glass slide (fig. 126). On the lower 

 surface of the slide and accurately at the centre of the ring, a circle of 

 Indian ink 1-3 mm. diameter is painted, for the purpose of attaining 

 dark-ground illumination. The author uses Reichert's Microscope with 

 objective 3 and ocular 2 (magnification 50-76 diameters), completely 

 removes the collar and stop, and adjusts the black point of the object 



* Comptes Rendus, cxlv. (1907) pp. 112-15. 



t Zeitschr. wiss. Mikrosk., xxiv. (1907) pp. 97-103 (2 figs.). 



