4^0 



NATURE 



[December 15, 192 1 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communicatiotis.] 



The Action of Sunlight. 



Referring to Dr. Saleeby's letter in Nature of 

 December 8, p. 466, I ma> report that, in conjunction 

 with the late Marshall Ward, professor of botany at 

 Cooper's Hill, I carried out a number of experiments 

 at Liverpool near the end of last century on the effect 

 of light in slaughtering anthrax bacilli. Prof. Ward 

 prepared the cultures, covered them seriatim with a 

 quartz plate which I lent him, and then sent them 

 to me to be exposed to a suitably arranged arc light 

 through quartz lenses and a quartz prism, dis- 

 criminating the kind of light by its effect on fluorescent 

 paper pasted at the side. The experiments were never 

 properly published, though they are partly described 

 in Prof. Ward's memoir on the subject in the Phil. 

 Trans. Unfortunately, I do not possess the B series 

 to refer to, but Dr. Saleeby could easily find the 

 paper, and I think it would interest him. 



The general outcome, to my mind, was that the arc 

 light was more efficient than Liverpool sunlight, but 

 that the most effective parts of the spectrum were 

 two strongly phosphorescing bands in the ultra-violet, 

 not far from the visible portion, and not of such short 

 wave-length that clean air would be opaque to them. 

 Their wave-lengths were, in fact, about 3830 and 

 3250 tenth-metres. 



The unpolluted atmO|Sphere seems well adapted 

 to screen us from the really deleterious rays of exceed- 

 ingly short wave-length, while still allowing the 

 microbe-destroying rays to come through and do their 

 beneficent work. Oliver Lodge. 



Microscope Illumination and Fatigue. 



It is interesting to note from Mr. J. E. Barnard's 

 letter in Nature of December 8, p. 468, that other 

 workers have taken up the problem of variable illu- 

 mination for the microscope. But in criticising the 

 method outlined in these columns on November 17 

 Mr. Barnard seems to have overlooked the important 

 fact that a monochromatic light-filter is used, so 

 that the question created by the shift of the dominant 

 radiation does not arise. This shift— which in its 

 simplest expression amounts to a reddening of the 

 light as the temperature of the source is decreased— 

 was fully recognised when the method of regulation 

 was originated, and in practice is a distinct advantage, 

 since the apparent decrease of illumination of the 

 field, when a filter is used which passes only a small 

 band of the spectrum, is greater than that of the light 

 source alone, where the full spectrum is present. 

 Hence any increase of the resistance of the circuit 

 doubly decreases the light in the field. 



The neutral wedge device described bv Mr. Barnard 

 is very ingenious, and if it can be controlled from 

 the front of the working bench by a long actuating 

 spindle should be of the greatest use when mono- 

 chromatic light is not desired. It is not easy to put 

 an arm round to the front of the microscope to make 

 an adjustment without moving the eye from the eye- 

 piece, especially when a drawing-table and other acces- 

 sories are in use. The adjustable resistance can, of 

 course, be put wherever it is most convenient to the 

 hand, and its use in a short time becomes almost sub- 

 conscious. 



NO. 2720, VOL. 108] 



It woqld be of interest to know whether in Mr. 

 Barnard's experience he finds that there is any in- 

 creased visibility of the finer details when variable 

 illumination is used. This is markedly the case in 

 cytological preparations and in the examination of 

 cotton hairs. 



The use of monochromatic light, so strongly advo- 

 cated by Dr. Spitta, raises several questions, and is 

 open to criticism where double staining methods are 

 used, though with two filters all details can usually 

 be made out. There was a half-promise in Zeiss 's 

 1913 catalogue of monochromat objectives. These 

 were put on the market some years before the war, 

 corrected for monochromatic ultra-violet light, and an 

 admirable description of their use was given by 

 Mr. Barnard in Nature of November 28, 1920, but 

 there would be a real use for monochromats for 

 visible light of a definite short wave-length if the 

 attention now given to apochromatism could be trans- 

 ferred to flatness of field. H. J. Denham. 



Shirley Institute, Didsbury, Manchester, 

 December 9. 



Tin Plague and Scott's Antarctic Expedition. 



It will be recalled that a chief contributing cause 

 of the failure of Scott's party to get back to their 

 base was the "leakage" of the fuel oil that was 

 stored in tin cans at the dep6ts along the line of the 

 return march from the Pole. The oil-cans as found 

 were apparently intact and "without hole of any 

 kind," and it was therefore thought by some that the 

 oil had evaporated through the "stoppers." Scott 

 himself, however, wrote in his last message: "We 

 should have got through in spite of the weather but 

 for . . . and a shortage of fuel in our depots, for 

 which I cannot account, and ..." 



Now it has been clearly understood since 1899, by 

 reason, chiefly, of the continued investigations of the 

 Dutch chemist, Prof. Ernst Cohen, and his colla- 

 borators, that ordinary tenacious white tin is no 

 longer stable below 18° C, but may change into a 

 modification that appears grey and pulverulent. This 

 change, long since observed in organ-pipes and other 

 articles in very cold climates, is referred to by Prof. 

 Cohen as tin plague, and spreads fastest, according 

 to him, at about -50° C. Ever since reading Scott's 

 diarv in 19 13 the present writer has in his teaching 

 been in the habit of suggesting that certain of Scott's 

 paraffin cans contracted tin disease, thus exposing the 

 underlying iron, in spots at least, to the danger of 

 chemical action and so of becoming "pin-holed," 

 with the possible aid of electrolytes present in traces 

 from the process of refining the oil. This rather 

 obvious suggestion has, however, not appeared in 

 your columns, nor was it known to Prof. Cohen, 

 who was lecturing here a few days ago, as he has 

 lectured elsewhere in this country and in Europe, on 

 tin plague and related matters. It was interesting 

 to learn from Prof. Cohen that he had experience 

 of preciselv the same phenomenon in the case of 

 canned foods stored at rather low temperatures. 



Alan W. C. Menzies. 



Princeton University, Princeton, N.J., U.S.A., 

 November 2^ 



The Dispersal of Snails by Birds. 



I WAS present at a meeting of the Malacological 

 Societv last May when Dr. Boycott read a very 

 interesting paper, in which he showed how the small 

 snail Balea perversa occurred on trees, walls, and 

 rocks, but not on the ground. The question arose 



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