February i6, 1893] 



NATURE 



365 



in the backwaters and marginal pools of the lower reaches of 

 larger rivers ; we are not only entitled, but we are bound to 

 consider this to have been the case in Greenland, and to base 

 our estimate of its climate in the lower tertiaries upon this view 

 and no other. Now what geologists and physicists ought to do, 

 and what they resolutely won't do, is before going farther afield 

 for cause and effect, to take the map of the world on Mercator's 

 projection, and consider how far, if the Atlantic were a closed 

 ocean to the north, as we know it must have been, the required 

 climatic conditions would be produced. The difference between 

 the arbutus nooks of Ireland on the one side and the desolation of 

 Labrador on the other is brought about solely by ocean currents. 

 At the period of the Greenland floras the arctic currents were 

 excluded, and consequently the whole Atlantic basin was filled 

 with the circulation of equatorial and temperate waters only. 

 The distribution of plants and animals renders it extremely 

 probable that during much of the tertiary period, the antarctic 

 waters were equally excluded from the Atlantic by land con- 

 necting Africa and South America. What, under these circum- 

 stances, would happen to the climate of the Atlantic littoral ? 

 It would, it appears to me, be more philosophical to dispose of 

 this question, which is supported by a weight of evidence, before 

 invoking shifting of the earth's axis, or other hypothetical causes 

 supported by none. J. Starkie Gardner. 



London, February 13. 



An Optical Phenomenon. 



In Nature, vol. xlvii. p. 303, you mention that " a beautiful 

 optical phenomenon, which has not yet been satisfactorily ex- 

 plained, is described by M. F. Folic in the Bulletin of the 

 Belgian Academy." From what follows, it is evidently the 

 same as that described in Tyndall's " Glaciers of the Alps " 

 (Murray, i860), p. 177 et seq. Tyndall gives a description of 

 it in a letter from Prof. Necker to Sir David Brewster, from 

 which I quote the following: — "You must conceive the 

 observer placed at the foot of a hill between him and the place 

 where the sun is rising, and thus entirely in the shade ; the 

 upper margin of the mountain is covered with woods, or de- 

 tached trees and shrubs, which are projected as dark objects on 

 a very bright and clear sky, except at the very place where the 

 sun is just going to rise ; for there all the trees and shrubs bor- 

 dering the margin are of a pure and brilliant white, appearing 

 extremely bright and luminous, although projected on a most 

 brilliant and luminous sky. You would fancy you saw these 

 trees made of the purest silver." 



Prof. Necker says that he saw it at the Saleve, which is not 

 so high above the Lake of Geneva as some of our British 

 mountains above the sea, and has no permanent snow near it ; 

 so that M. Folie's suggestion, that it is due to light reflected 

 from snow, must be wrong. I have seen it from the Konig-See, 

 near which I believe there is no permanent snow. 



This appearance is always to be seen under the circumstances 

 described, when the sky is clear and bright enough. I had 

 read of it in Tyndall's book, and when in the Alps I sought for 

 and found it. I have often seen a distant approach to it pro- 

 duced by furze bushes, quite near, seen against sunlight, and by 

 leaves against moonlight. Joseph John Murphy. 



P.S. — Ruskin somewhere describes this phenomenon. 



Belfast, February 6. 



Foraminifer or Sponge ? 



A PAPER by A. Goes " On a peculiar type of Arenaceous 

 Foraminifer from the American tropical Pa.ci{ic,NeusznaAgasstzi, " 

 has just been published in the " Bulletin of the Museum of Comp. 

 Zoology.at Harvard College," vol.xxiii. N0.5, in which the author 

 describes some remarkable forms dredged by the Albatross ex- 

 pedition in the Pacific of Central America. They are supposed to 

 be foraminifera, are of leaf-like shape, measure up to 190 mm. 

 in breadth, and are marked by concentric lines of growth. Their 

 interior shows a stroma, consisting of fine chitinous threads, 

 enclosing sand and debris of shells. Without wishing to re- 

 capitulate all the various points of structure, I will only say that 

 there can be no doubt that these forms belong to Hseckel's deep 

 sea keratosa (see Challenger report, vol. xxxii.) from the tropical 

 Pacific, and I should think that Neusina Agassizi is identical 

 viii\i Stannophyllumzonarium, Hseckel. I happen to have here 

 a Challenger specimen of this latter species, kindly lent to me 

 by the Manchester Museum, and its microscopic examination 

 convinces me of the identity of the two forms. 



University Collie, Liverpool. R. Hanitsch. 



NO. I 2 16, VOL. 47] 



Unusual Origin of Arteries in the Rabbit. 



Towards the close of last month Prof. W. N. Parker re- 

 ported in your columns an abnormality in the veins of the rabbit, 

 and although the same interest does not attach to it, it may be 

 worth while recording an unusual arrangement of the vessels 

 arising from the aortic arch. In the case which has just come 

 under my notice, the two carotids arise together from the arch, 

 at the point usually occupied by the innominate artery, while the 

 right subclavian artery arises beside the left subclavian, which 

 occupies the usual position. Philip J. White. 



University College of North Wales, February 7. 



Holmes's Comet. 



On February ir, loh. to loh. 35m., I re-observed this object 

 with powers of 40 and 60 on my newly-silvered lo-inch reflector. 

 The comet was in the same field as )3 Trianguli and south pre- 

 ceding that star. I found it fairly conspicuous. The nucleus, 

 or brighter portion ©f the head, presented a distinctly granulated 

 appearance. Applying a power of 145, single lens, I saw that 

 it really consisted of a number of very small knots of nebu- 

 losity, so closely approximating the stellar form that they might 

 readily have been mistaken for one of the very faint, barely 

 resolvable clusters in which the components are only to be caught 

 by glimpses. The multiple nucleus was involved and surrounded 

 with feeble nebulosity, and a faint tapering tail flowed from it in 

 a N.E. direction. I believe that outlying this there was an ex- 

 cessively faint fan-shaped tail, but could not be absolutely 

 certain. 



The sky was not good, being lighter than usual, with suffused 

 mist. On February 12, at loh. 15m., I picked up the comet 

 again, but details were invisible, owing to the veil of thin cloud 

 overspreading the N.W. sky at the time. 



Bristol, February 13. W. F. Denning. 



HELMHOLTZ ON H BRINGS THEORY OF 

 COLOUR. 



THE following translation of the critical account given 

 by von Helmholtz of the colour-theory of E. Hering, 

 in the new edition of his Handbuch der Physiologischen 

 Optik, commencing at page 376, has been made by Prof. 

 Everett for Nature. The translator aims at clearness 

 rather than literal rendering, and three obvious misprints 

 in the paragraph on the transformation of coordinates 

 have been corrected. " Lambert's colour-pyramid " is 

 another name for the '* cone of colour " described in Max- 

 well's papers and in § 1074 of Everett's " Deschanel." 



This much-talked-of theory is a modification of Young's 

 theory, which, by the choice of other fundamental sen- 

 sations, endeavours to give better explanations of what it 

 regards as immediate facts of internal observation. It 

 assumes three elementary sensations, related to three 

 different parts of the nerve-apparatus or " visual sub- 

 stance." Two at least of these physiological processes 

 exhibit the opposition of positive and negative. One of 

 the three " visual substances " gives in the condition of 

 excitement the sensation of white, and in the condition 

 of rest the sensation of black. The second gives the two 

 sensations of blue and yellow, which are accordingly 

 designated " opposed colour-sensations." The third 

 gives the other pair of "opposed colour-sensations," red 

 and green. But by "red" is denoted not the colour 

 usually so called, but the complementary of green, which 

 is purple. 



It is possible to specify "elementary sensations" (in the 

 sense in which we have previously defined the term) 

 which would correspond to Hering's elementary sensations, 

 and would be capable of giving by their combination all 

 other colour-sensations. If we take three rectangular 

 axes of coordinates, x, y, z, as the edges of Lambert's 

 colour-pyramid, .r corresponding to red, y to green, and 



