April 



1913] 



NATURE 



1 1 1 



definite and bright when the amount was large, and 

 occasionally disappearing in part or even completely 

 as the atmosphere cleared. Sometimes the whole halo 

 became uniformly faint, while at other times portions 

 of the circumference disappeared. The major axis 

 was vertical, the moon being about on the meridian. 

 The axes were estimated to be about 7 and 4 re- 

 spectively, but no careful measures were made. The 

 colour, if any, was very slight. The moon was at 

 first quarter." 



None of my astronomical friends to whom I have 

 described these elliptical halos has ever seen one. 

 They must be rare phenomena, and well worth re- 

 cording. Frank Schlesinger. 



Allegheny Observatory, February 24. 



The halo appears to be that known as " Hall's halo," 

 but the diameters given are rather less than those 

 observed by Hall. The phenomenon is mentioned in 

 Pernter's " Meteorologische Optik " (p. 262), and an 

 explanation is suggested by him on p. 381 of that 

 work. [Ed. Nature.] 



The Reflection of X-Rays. 



In continuation of the experiments of Mr. W. L. 

 Bragg (Nature, December 12, 1912, p. 410), 1 have 

 investigated the reflection of X-rays by mica. Mr. 

 Bragg finds one reflected beam, while Messrs. Hupka 

 and Steinhaus (Nature, March 6, 1913, p. 10) find two 

 beams. Using a parallel pencil and an angle of 

 incidence of 70°, I find no difficulty in photographing 

 five beams emerging from the "incident" side of the 

 mica, of which that obeying the ordinary laws of 

 reflection is the most obvious. 



From the "transmitted" side of the mica sheet 

 there are certainly no fewer than thirty distinct beams 

 apart from the intense primary beam which has 

 passed through the crystal (0-33 mm. thick) without 

 much absorption. The plane of the mica sheet was 

 perpendicular to that of the photographic plate. In 

 .the reproduction given below, the intense black spot 



is produced by 

 the transmitted 

 primary b e a m, 

 while beneath 

 it is seen another 

 circular patch due 

 to the ordinary 

 reflected beam. 



The greatest 

 photographic in- 

 tensity occurs in 

 those transmitted 

 beams which have 

 suffered the least 

 deviation, the 

 ordinary reflected 

 pencil being feeble 

 in comparison with 

 some of them. 

 It will be evident that the transmitted pattern is 

 analogous to that obtained by Messrs. Laue, Fried- 

 rich, and Knipping (Nature, November 14, 1912, 

 p 306), using a pencil of X-rays falling normally on a 

 crystal of zincblende. Repeating mv experiments, 

 using a normal pencil, a transmitted pattern is ob- 

 tained similar to theirs. 



Besides giving rise to numerous pencils in definite 

 directions, the mica sheet exhibits the ordinarv inci- 

 dent and emergent scattering. It is well known that 

 this effect is small in the plane of the radiator. This 

 is borne out in all the negatives which exhibit general 

 fogsjing, except along a line which represents the line 

 of intersection of the photographic plate by a plane 

 NO. 2266, VOL. 91] 



containing the mica sheet. This line is represented 

 in the diagram by the broken line. Similar results 

 are obtained using rock salt and galena. 



Since the photograph described above is unsuitable 

 for reproduction by a half-tone block, I have been 

 obliged here to substitute a diagrammatic copy for it. 



H. B. Keene. 

 Physics Department, University of Birmingham, 

 March 15. 



The Presence of Protozoa in Soils. 



Up to the present, so far as I am aware, the only 

 method of demonstrating the presence of Protozoa 

 in soils has been by cultures. This method, of course, 

 leaves untouched the really important question as to 

 what Protozoa are leading a trophic existence at any 

 given time in a soil sample, since many of the forms 

 found later on in cultures may be derived from cysts. 

 In these circumstances I thought it might be of 

 interest to direct attention to a method by which the 

 presence of Protozoa in the trophic stage in the soils 

 can be readily demonstrated, even though this method 

 from a quantitative point of view probably gives low- 

 results. 



A small quantity of the soil to be investigated is 

 mixed, as soon as it is collected, with about an equal 

 volume of picric aci'd. The mixture is then placed in 

 a wide dish and carefully stirred, so that the organisms 

 on the surface films between the soil particles are free. 

 If the mixture is then allowed to stand for a time 

 it will be found that most of the bacteria, diatoms 

 and Protozoa that were present come up to the surface 

 film. The coverslips, cut according to the method 

 which I have previously described, can then be floated 

 on the film, and then placed in tubes containing 

 corrosive. These cover-slips can then be handled as 

 though thev were ordinary smears. The best method 

 of staining seems to be to stain for some time in 

 strong acid haemalum, followed by eosin. I have 

 tried mixing the soil in the first instance with other 

 fixatives in the place of picric acid, but have not ob- 

 tained such good results. By tfiis method perfectly 

 clean preparations, showing large numbers of amoeba 

 and flagellates, have been obtained from a six weeks 

 old bed, which had been used for growing seedling 

 cauliflowers. These have been used to compare the 

 active fauna of such a soil with the fauna derived 

 from the same soil in cultures. 



As might be expected, it has been found that the 

 prevalence of any given Protozoon in the cultures is 

 not an indication of its prevalence in the trophic stage 

 in the soil, though by varying the methods of culture 

 it has been found possible to cultivate all the Protozoa 

 that have been found by the above method leading a 

 trophic life in this soil at the date of fixation. 



C. H. Martin. 



The Hill, Abergavenny, March 19. 



Jelly-fish of the Norquane River. 



The discovery of a jelly-fish in the northern water- 

 shed of the Limpopo will be of some interest to 

 zoologists. 



During the new year holidays, while making zoo- 

 logical collections in the Bembezi district (thirty miles 

 north-east of Bulawayo), I noticed some jelly-fishes 

 in a pool of the Norquane River, a tributary of the 

 fourth degree of the Limpopo. 



With the scanty literature at my disposal, it is not 

 possible at present to identify or determine it as a new 

 species, but judging by the figures in Lankester's 

 " Treatise on Zoology " and in the " Cambridge 

 Natural History," and also by Mr. Moore's statement 

 ("The Tanganyika Problem") that Limnocnida tan- 

 ganyikae varies in size from that of a shilling to 



