470 SUMMARY OF CURRENT RESEARCHES RELATING TO 



so that they appeared as if mounted on a colourless fluid. Cover-glass 

 preparations, from broth cultures, before being stained, were dipped in 

 5 p.c. acetic acid for ten seconds and washed in water, to which a fow 

 drops of ammonia had been added to more rapidly remove the acid. 



Resolution of Amphipleura pellucida.* — Mr. A. A. Merlin states 

 that, by increasing the solid axial illuminating cone from |- to f of the 

 total aperture of a dry apochromatic, 4 mm., with a 27 eye-piece, he 

 was able to hold for short intervals, with slightly averted vision, the 

 transverse strire on Amphipleura pellucida, first when it was mounted 

 in realgar, then dry, and finally in balsam. The following difficult 

 valves in Moller's balsam-mounted type slide were in the same manner 

 also resolved, viz. the dots on the hoop of Navicula major ; Navicula 

 crassinervis ; Grammatophora oceanica = G. subtilissima ; and Nitzschia 

 sigmatella. The author has not seen the most difficult structural features 

 with a lesser cone, and says that the closing down of the cone, while 

 greatly strengthening the contrast of the coarser, causes the finer detail 

 to disappear altogether, and materially reduces the separating power of 

 the objective. An acetate of copper screen was used throughout these 

 investigations. 



Origin of certain Colour Phenomena typically shown by Actino- 

 cyclus Ralfsii.l — J. Rheinberg gives a very satisfactory explanation of 

 the colour effects seen when certain diatoms, notably A. Balfsii, are 

 viewed by transmitted light, and by an objective aperture not greater 

 than 0*45 N.A. Experiments showed that the phenomena were not 

 connected either with polarising or with diffraction effects. The 

 thickness of the diatom placed the possibility of film colours out of the 

 question. Ultimately " interference " suggested itself as the cause. 

 Mr. Rheinberg's diagrams show how the light rays, in passing through 

 the inequalities on the surface of the diatom, or through its apertures, 

 or through the mounting medium, are sufficiently retarded in various 

 parts to produce " interference " fringes. Other diatoms of suitable 

 thickness will show similar phenomena, and for a mount of any specific 

 medium a minimum and a maximum limit can be assigned. A great 

 many diatom valves are of a thickness below the necessary limit, or only 

 slightly exceed it, and assume a pale yellow-brown tint (that being the 

 first colour in the series) e.g. Pleurosigma angulatum, mounted dry. The 

 limits vary with the difference in refractive index of the diatom silex 

 and its medium. The greater the differences, the smaller is the mini- 

 mum limit for the occurrence of colour. The technical name for the 

 whole phenomenon is " the colours of mixed plates." J 



Progress in Metallography.§ — Henri le Chatelier proposes an im- 

 proved method of preparing polishing powders. The best material, as 

 far as speed of polishing is concerned, he finds is alumina, prepared by 

 calcining ammonium alum ; but oxide of chromium, obtained from the 

 combustion of bichromate of ammonium, answers fairly well in the 

 treatment of iron and steel, and is better than alumina for soft metals 



* Journ. Quek. Micr. Club, viii. (1901) pp. 1-6. See also ante, p. 397. 

 t Tom. cit., pp. 13-21 (11 figs.). I Preston, Theory of Light, p. 205. 



§ Bull. Soc. d'Encouragement, Sept. 1900; aud Nature, lxiii. (1901) pp. 232-3 

 '8 figs.). 



