542 



NATURE 



[July 24, 1913 



Zealand, 32 in the Antarctic, and 31 in South Georgia, 

 the latter being evidently, from the phytogeographic 

 point of view, a half-way house on the road from 

 subantarctic America to the true antarctic area. 

 Moreover, practically half of the antarctic species are 

 common also to the arctic regions. 



Of the 106 antarctic lichens, sixty-nine are crus- 

 taceous, eighteen foliaceous, and nineteen fruticulose 

 species; of these, the numbers found in subantarctic 

 America are respectively sixteen, five, and eleven. 

 Of the sixty-seven species found only in the true ant- 

 arctic area, forty-nine are crustaceous, ten foliaceous, 

 eight fruticulose. The subantarctic American 

 lichen flora includes 366 species, while 740 species 

 have been enumerated for New Zealand; of the 

 species common to the two regions 50 per cent, are 

 fruticulose, 30 per cent, foliaceous, and only 20 per 

 cent, crustaceous. The affinity of the subantarctic 

 American and New Zealand lichen floras lies mainly 

 in the fruticulose lichens, which are the oldest and 

 probably the least variable forms. The encrusting 

 species are more variable and have adapted themselves 

 more readily to local conditions, thus giving rise to 

 new species. An interesting point arises from a com- 

 parison with northern lichen floras. The arctic area 

 had nearly 500 lichens, of which 72 per cent, are found 

 in Tyrol. Thus the relation of arctic to alpine lichens 

 is much greater than that of subantarctic American 

 to New Zealand species, indicating that the latter are 

 further from the point of common origin. 



Dr. Darbishire raises the interesting question of 

 the resistance of cold by lichens, and suggests some 

 simple experiments which might be made on lichens 

 in the verv coldest regions. For instance, it would 

 be of the greatest importance to determine the amount 

 of water contained in the lichen thallus at various 

 times and seasons. In what condition are lichens 

 during the long winter? At what temperature does 

 assimilation commence? It is of little use to try 

 experiments on plants in warmer climates, if we wish 

 to ascertain how these small plants can live under 

 the adverse conditions prevailing in the arctic and 

 antarctic regions. 



Lichens are found everywhere on the outer limits 

 of vegetation, and their chief ecological distribution 

 factor is their power to become quite dry and yet 

 remain alive. No doubt it is this property which 

 enables them to spread slowly but surely into the 

 bleakest and most inhospitable regions. ' They are 

 making their way towards the north and south poles, 

 and so far they have been beaten in their race only 

 by the perpetual covering of snow. There is little 

 doubt that if bare rocks are found in the neighbour- 

 hood _ of the poles themselves, lichens will be found 

 growing there. 



Dr. Darbishire's memoir is illustrated bv three 

 double plates of beautifully reproduced photographs, 

 depicting the new species brought back by the 

 expedition. F. C. 



APPLICATIONS OF POLARISED LIGHT. 

 r\N November 30, 1812, just above 100 years ago, 

 ^J the French physicist Biot communicated to the' 

 Institute of France a memoir "on a new kind of oscilla- 

 tion which the molecules of light experience in travers- 

 ing certain crystals." In this paper, which extends 

 over 371 pages of the printed memoirs, the 

 phenomenon of "rotatory polarisation" was described 

 for the first time. This phenomenon depends on the 

 property which certain substances possess of taking a 

 beam of polarised light and imparting a twist to the 



1 Discourse delivered at the Royal Institution on Friday, April 18, by Dr. 

 T. M. Lowry. 



NO. 2282, VOL. 91] 



plane of polarisation : the beam of light enters with 

 all the vibrations compressed, say, into a vertical 

 plane ; it emerges apparently unchanged, but careful 

 examination shows that the component vibrations are 

 no longer vertical, but inclined either to the right or 

 to the left. The importance of this discovery to 

 physicists and to crystallographers was immediately 

 obvious. In our own generation its fertility has been 

 realised also by chemists, who have found in the 

 polarimeter an instrument which promises to render 

 to the science services not less notable than those 

 which have been accomplished with the help of the 

 spectroscope. 



A. — Sources of Polarised Light. 



If one were to ask what progress had been made in 

 the facilities for applying polarised light to the study 

 of chemical and physical problems, the answer would 

 be twofold. On one hand it must be acknowledged 

 that the " Iceland spar," by means of which Huyghens 

 in 1678 first detected the polarisation of light, is still 

 the best substance for producing this effect. But the 

 increasing demand for the spar has not been accom- 

 panied by any corresponding increase in the supply, 

 and large clear pieces of the mineral are becoming 

 increasingly difficult to procure. It may indeed be 

 doubted whether large polarising prisms such as those 

 which have been handed down as heirlooms at the 

 Royal Institution could now be purchased at any 

 price, in view of the "spar-famine" which has pre- 

 vailed for some years. 



Considerable advance has, however, been made in 

 the direction of improved methods of illumination. 

 The solar light, which figured so largely in the experi- 

 ments of the earlier workers, is too precarious to 

 satisfy the ardent worker of to-day, and in any case 

 could render no direct assistance in illustrating a 

 Friday evening discourse. When Faraday, on Friday, 

 January 23, 1846, delivered his discourse on the mag- 

 netisation of light to an audience of 1003 persons, the 

 source of light in the experiments which he described 

 was an Argand gas-burner. Prof. Silvanus Thomp- 

 son in 1889 was able to use the electric arc, which 

 was then just beginning to come to the front as a 

 commercial illuminant. With this unrivalled source 

 of light he was able to show for the first time in a 

 public lecture a large number of the properties of 

 polarised light which had been reserved hitherto for 

 individual observation in the laboratory. The remark- 

 able effects which are seen when light of one single 

 colour or wave-length is substituted for white light 

 were shown by Spottiswoode in 1878, with the help of 

 a powerful sodium-lamp which had been devised by 

 Sir James Dewar. His lecture was aptlv described as 

 "A Nocturne in Black and Yellow." 



During several years I have taken a special interest 

 in seeking to discover other sources of monochromatic 

 light for use, in experiments on polarisation, and have 

 been particularly concerned to proclaim the merits of 

 the_ mercury arc as an illuminant for evervdav use in 

 optical investigations. 



The Mercury Arc. 

 The spectrum of the light produced by passing an 

 electric discharge through mercury vapour was de- 

 scribed by Wheatstone in 1835 in a report to the 

 British Association on the prismatic decomposition of 

 electric light; but it was not until twenty-five years 

 later that a real mercury-lamp was invented by Prof. 

 Way. This consisted of an intermittent jet of mercury 

 which was directed into a cup half an inch below. 

 The current from a battery of Bunsen cells was passed 

 through the jet and developed an intense light. The 

 snectrum of the light was examined by Dr. J. H. 

 Gladstone, and described in a paper on the electric 



