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NA TURE 



[Feb. 26, 1885 



tion for a nearer point than that on which attention is directed — 

 a kind of spasmodic myopia, and, as such, would disappear when 

 the power of accommodation was paralysed by atropine. On 

 the other hand, it may not be myopia at all, the improvement 

 given by the weak concave lens being perhaps due to the con- 

 traction of the pupil, which would occur along with the accom- 

 modation necessary to neutralise the effect of the glass. If this 

 were the case, the improvement would also take place by the 

 use of a suitable diaphragm held in front of the eye. Still 

 another possible explanation suggests itself, viz. that the new 

 dioptric combination made up of the concave lens and partially ac- 

 commodated crystalline might introduce conditions of chromatic 

 and spherical aberration which were more favourable to distinct 

 vision. The disturbing effects of such aberration are probably 

 greatly neutralised by the arrangement of the retinal elements, 

 but the degree of the neutralisation is, not unlikely, dependent 

 on the amount of absolute and relative illumination of contiguous 

 elements. Geo. A. Berry 



Edinburgh 



The Fall of Autumnal Foliage 



The paper by Mr. Sorby in Nature for December 4, 1884 

 (p. 105) opens up an unpursued inquiry into the cause of leaves 

 falling in autumn. While Mr. Sorby has had his attention 

 drawn to the subject by looking at the actual trees and leaves 

 "of the fine display of autumnal tints which we have lately 

 seen " in England, there is much of both positive and negative 

 evidence to be drawn in two extreme directions — the tropics and 

 the pole. 



Being, in the year 1881, home from India, where, it is not 

 necessary to say, nearly all the trees retain their green foliage 

 throughout the year, the writer indulged in a long curiosity to 

 see the counties of Caithness, Orkney, and Shetland He went 

 there with reference to the luminosity, which reaches its maxi. 

 mum in them for Great Britain, and is very marked and exceed- 

 ingly striking and beautiful as a feature all over the north of 

 Scotland in the month of June, when it is daylight all through 

 the hours of night, sufficiently clear for reading distinct print 

 at twelve o'clock midnight. 



A peculiarity of Caithness and the Orkney and Shetland 

 Islands is that no forest-trees can be got to grow. Setting on 

 one side a remark "that it was because nobody had tried," the 

 suspicion had already occurred to my mind that there must exist 

 some other causes than those usually asserted — the high sea 

 winds, bleakness in winter, and extreme cold — for this want of 

 trees. 



Any one who has been much in the north of Scotland, and is 

 at all acquainted with the optical sciences, cannot fail to have 

 noticed the immense amount of polarised light there is from the 

 sky ; almost all the diffused daylight, except for an hour or two 

 in the middle of the day, being plane or elliptically polarised. 



The attention of readers of Nature may with advantage be 

 specially directed to the possibility, from the phenomena of the 

 north, that leaves fall in autumn from trees growing above a 

 certain latitude — about 30 — through loss of vitality in the more 

 or less highly polarised light. 



The first thing a traveller from India notices in Alexandria is 

 the American fall of the leaves in the Grande Place, or, as a 

 fellow-passenger once put it, pointing to these, "It is here trees 

 fir^t become deciduous " It is worth being remarked that, not 

 until reaching Cairo or Alexandria, can sun-protection be done 

 without. 



So far Mr. Sorby has to refer to the action of light in the last 

 resort, as he says, with regard to leave , "slight frosts reduce 

 their vitality in such a manner, that the chlorophyll is changed 

 by the action of the light into a red product." 



Chlorophyll is composed of carbon, hydrogen, oxygen, and a 

 trace of iron. Chemically it is C 18 H„ N„O 3 + 18 , resulting 

 from the action of carbonic acid and ammonia on a fat, C 8 H 14 G, 

 under the influence of light, as given by a different authority ; 

 but the composition of its products and combinations have not 

 been traced. Still there is almost every constituent of the 

 animal frame present except the earthy salt?, and it must be a 

 substance very sensitive to rays of light, or to what light 

 probably is, electro-magnetic forces. 



The weakening of the plant is supposed by Mr. Sorby to 

 have occurred, for the leaves of a tree to have lost the vitality 

 which counteracted the chemical degradation of the chlorophyll. 

 Now in India or Ceylon, if a stalk were injured, the leaves 



would wither into brown. Trees remain, however, when living, 

 constantly green, the leaves dropping off gradually one by one 

 almost, and are immediately replaced. Indian leaves of trees 

 are much thicker, and more of the texture of parchment than 

 those of foliage in European countries, and the phenomena of 

 change can be studied in evergreens without going there, Indian 

 observation merely serving to draw attention that might not 

 otherwise be given to the matter. 



The Rothamsted experiments of Sir J. B. Lawes and Dr. Gil- 

 bert, F.R.S., bear closely on the question. They found (Swansea, 

 18S0, address) that plants assimilate chlorophyll not only during 

 but a small portion of the year, but the action is limited to the 

 hours of daylight, while during darkness there is rather loss than 

 gain. The experiments, however, both there and in Norway 

 by Prof. Schiibeler, were made in ordinary unpolarised solar or 

 electric light. 



On the other hand, in India the light is intense owing to its 

 tropical position, and, from the altitude of the course of the sun, 

 very slightly polarised. It is only for an hour at dawn and 

 another hour of sunsetting that the Indian is at all the same sort 

 of daylight that it is in England. It accords with the Rotham- 

 sted and Norwegian experiments under the continuous exposure 

 of vegetation to daylight and electric illumination during the 

 night that the trees in India are large and evergreen. Of course 

 in time leaves have done their work and fade, but as they have 

 not been unfolded simultaneously, they drop off gradually in 

 batches. 



Where, accordingly, the light is polarised, trees are scarce or 

 absent, mown by a swathing light ; and in the tropics, where 

 there is little polarisation, they are luxuriant, and green all the 

 year round. 



This is not inconsistent with fact. To begin with, plane 

 polarised light has half the intensity of ordinary white light, the 

 set of vibrations at right angles to the plane of polarisation 

 being absorbed in the reflecting matter of the sky. Besides, 

 circularly or elliptically polarised light must largely prevail, to 

 judge from the metallic glow there is on the Pentland Firth, 

 Orkney, and Shetland in midsummer, and what effect circularly 

 polarised light has on the assimilation of carbon in the leaves of 

 plants and decomposition of chlorophyll is unknown. 



At any rate, Caithness, and the northern islands have a 

 number of hours in the daytime of a wintry darkness, and 

 scarcely any light in the summer months and its long days that 

 is not polarised. From this cause, which could in the leisure of 

 their winter be put in arithmetical units of force, combined 

 with cold winds and a thin soil, without alluvial deposits, resting 

 on stone, it is no wonder that, though the inhabitants are not 

 strangers to the pathos of the fall of the leaf, the Caithness-shire 

 landscape, and the sward and heather of Orkney and Shetland 

 are lustrous day and night with polarised light, and bare of 

 autumnal foliage. A. T. Fraser 



India, January 22 



Erosion of Glass 

 In reference to the letter of Dr. Ord in last week's Nature, 

 glass is by no means proof against the action of either acids 

 or alkalies, indeed its resisting seems to depend merely on its 

 colloidal, at any rate non-permeable, nature. It may not be 

 generally known that water alone very rapidly acts on glass, 

 especially when it is in a finely divided state, extracting both 

 alkalies and silica in quantity. It would be rash to put down 

 the action of substances on glass to " molecular coalescence " to 

 the exclusion of chemical action, or under the idea that acids or 

 fluorine are necessary to etch glass. Alkaline salts, especially 

 phosphates, act, either wet or dry, very vigorously on glass. 

 One class of salts, the potassium salts of phenol sulphonic acids, 

 have been noticed to literally tear a glass bottle in pieces, whilst 

 crystallising out of an acid solution. Ordinary gum is often 

 acid in reaction ; but the ordinary mechanical action of sticking 

 and then contracting is probably quite sufficient to cause an 

 abrasion or etching, especially with soda-glass. This purely 

 mechanical action is often noticed in the distillation of tarry 

 substances which solidify at a high temperature, the whole 

 interior surface of the retort being torn off and cracked in all 

 directions. W. R. II. 



A Lantern Screen 



The optical lantern has come to be so much used for scientific 

 and educational purposes, that you may perhaps think it useful 



