2o6 



NATURE 



[May io, 19 17 



With reference to the recent correspondence in the 

 columns of Nature upon the frequency of snowfall in 

 London during the past winter, it may be of interest 

 to state that at the Hampstead Scientific Society's 

 observatory at the extreme summit of Hampstead Heath 

 (453 ft.) some form of snow or sleet was recorded on 

 as many as sixty-one days, nineteen of these being in 

 January, and thirteen both in March and in April. 



In this country most of our snow comes with winds 

 from N. and N.E., and hence it need scarcely seem 

 a matter for wonder that a place, such as Hamp- 

 stead, situated to the north of the artificially heated 

 metropolitan zone, should sometimes receive in the 

 form of snow or sleet precipitation which reaches the 

 southern suburbs as rain. Apart from the difference 

 in height, the heating of the north or north-east cur- 

 rent in its traverse over the city would account for the 

 discrepancy mentioned by Mr. Bonacina. 



This is, to my mind, a more probable explana- 

 tion than that the observer at Wandsworth Common, 

 who has assisted with distinction in the upbringing of 

 modern meteorology almost since its birth, could fail 

 to recognise snow or sleet, in whatever form it may 

 have fallen. E. L. Hawke. 



May 5. 



A Canvas-attacking Fungus. 



So many inquiries have been made from strangely 

 diverse sources, especially since' fhe outbreak of war, 

 concerning black spots which appear on bell-tents, 

 sails, aeroplane and airship fabrics, etc., that it 

 seemed desirable to write the present note principally 

 to direct attention to a paper by F. Gu^guen in 

 Comptes rendus, vol. clix. (1914), p. 781, " Sur I'alt^ra- 

 tion dite ' piqure ' des toiles de tente et des toiles ^ 

 voile." The spots are caused by fungi which damage 

 the fabric, so that after some months it is easily torn. 

 The fungus hyphas grow on the surface of the fabric, 

 between the fibres and within the lumen of the fibres. 

 Gu^guen found that the fungi principally concerned 

 were the Pyrenomycetes, Pleospora infectoria and P. 

 herbarum, especially the former. These Ascomycetes 

 are also found in their conidial states, Alternaria 

 tenuis and Macrosporium commune, and other Muce- 

 dineae, Rhinocladium, Helminthosporium, etc., are 

 often associated with them. According to Gueguen, 

 the malady is scarcely ever due to accidental con- 

 tamination, but is caused by the development, in moist 

 warmth, of moulds already present in the newly manu- 

 factured fabric, commercial pattterns of the most 

 diverse origin being found almost all to contain 

 fungus spores. Practically all unbleached canvas is 

 affected, but that bleached with fiypochlorites, etc., 

 remains free — the glaucous colonies which are some- 

 times seen are due to Penicillium or Aspergillus de- 

 rived from the air, and almost invariably non-injurious 

 to the fabric. Gueguen holds that the fungi causing 

 the spots are those which grow on the dead stems of 

 the textile plant, which are introduced amongst the 

 fibres at the time of retting. The thick-walled hyphse 

 remain in a resting state in the dry canvas, and 

 resume vegetative growth when external conditions 

 become again favourable (humidity, warm confined 

 air). He considers that the best method of preven- 

 tion would be to sterilise the tow after retting, by 

 h'eat — steam under pressure, and then dry heat. Boil- 

 ing solutions of salts of chromium or copper would 

 also "serve, applied either to the tow or the fabric. A 

 suitable method of rendering awnings, etc., imper- 

 meable would be to immerse the fabric first in a 20 per 

 cent, solution of soap, and then in 8 per cent, copper 

 sulphate, each at boiling point. 



NO. 2480, VOL. 99] 



Similar black spots are very common on paper, and 

 are most commonly due to Alternaria, Stachybotrys,^ 

 and Chaetomium. S6e ("Sur les moisissures causant 

 I'alt^ration du papier," Comptes rendus, vol. clxiv.. 

 [1917], p. 230) has investigated the variously coloured 

 spots damaging paper, and believes that the 

 causative fungi are already present in the paper-pulp, 

 and probably come from the straw, fibre, etc., from 

 which the pulp is made. 



In the damaged fabrics examined by the writer the 

 perfect Pleospora stage has rarely been found, though 

 the Alternaria and Macrosporium conditions have been 

 frequent. Other Mucedineae, Cladosporium spp.,^ 

 Stachybotrys, Helminthosporium. etc., were also- 

 common. In certain cases fungi were found, how- 

 ever, which seem to be identical with species which 

 are known to occur in thte soil. A large number of 

 fungi are active cellulose destroyers ; many of these 

 occur only in the soil, and it seems probable that a 

 large proportion, if not most, of the cellulose destruc- 

 tion which goes on there is brought about by their 

 agency. Canvas left lying about on broken ground 

 would be almost certainly attacked by these cellulose 

 fermenters, given the suitable conditions for growth — a 

 very small portion of soil scattered over moistened 

 sterilised filter-paper gives rise to an amazing number 

 of fungus colonies. Although no experiments have 

 yet been undertaken in connection with this sugges- 

 tion, it is put forward for certa,in, more or less obvious- 

 reasons. . J. Ramsbottom. 



Department of Botany, 



British Museum (Nat. Hist.), London, S.W. 



Diffraction Phenomena in the Testing of Optical 

 Surfaces. 



In the Philosophical Magazine for February, 1917, 

 Lord Rayleigh has published an investigation of the 

 phenomena to be expected according to the wave 

 theory when an optical surface is tested at the focal' 

 plane by the well-known method due to Foucault, and 

 has shown that, even when nearly the whole of the 

 light is cut off by the advancing edge in the focal 

 plane, the boundaries of the aperture retain a very 

 marked brilliancy which is symmetrical about the 

 centre. 



An interesting question arises as to the manner in 

 which this effect (which has been shown by Lord 

 Rayleigh to be due to diffraction) would be modified 

 if the light is screened, not exactly at the focal plane, 

 but a little in front of, or behind, the focus. On 

 testing this at this laboratory it has been found that 

 the Rayleigh effect is still observed, but the edges of 

 the aperture on either side differ very markedly in 

 their brilliancy, one of the edges becoming several 

 times brighter than the other as the screen is removed 

 further and further from the focus. The explanation 

 of this asymmetry is apparently the fact that, as we 

 move away from the focus, the diffraction-pattern 

 which is screened gradually passes from the Fraunhofer 

 to the Fresnel class. Several series of photometric 

 comparisons of the brightness of the two edges have 

 been made at this laboratory, using a special 

 type of rotating sector photometer devised by C. V. 

 Raman (Phil. Mag., May, 1911) and constructed 

 by Hilger. 



The full mathematical treatment of the subject and 

 the detailed comparisons with the experimental results 

 will be published in due course. 



S. K. Banerji. 



Indian Association for the Cultivation 

 of Science, Calcutta, March 15. 



