backwaters and marginal pools of the lower reaches of rivers; we are not only entitled, but we are bound to er this to have been the case in Greenland, and to base imate of its climate in the lower tertiaries upon this view other. Now what geologists and physicists ought to do, hat they resolutely won't do, is before going farther afield se and effect, to take the map of the world on Mercator's ion, and consider how far, if the Atlantic were a closed to the north, as we know it must have been, the required e conditions would be produced. The difference between atus nooks of Ireland on the one side and the desolation of lor on the other is brought about solely by ocean currents. period of the Greenland floras the arctic currents were ed, and consequently the whole Atlantic basin was filled le circulation of equatorial and temperate waters only. istribution of plants and animals renders it extremely le that during much of the tertiary period, the antarctic were equally excluded from the Atlantic by land conAfrica and South America. What, under these circumwould happen to the climate of the Atlantic littoral? ld, it appears to me, be more philosophical to dispose of estion, which is supported by a weight of evidence, before ig shifting of the earth's axis, or other hypothetical causes ed by none. J. STARKIE GARDNER. lon, February 13. An Optical Phenomenon. ATURE, vol. xlvii. p. 303, you mention that " a beautiful phenomenon, which has not yet been satisfactorily exis described by M. F. Folie in the Bulletin of the Academy." From what follows, it is evidently the 3 that described in Tyndall's "Glaciers of the Alps y. 1860), p. 177 et seq. Tyndall gives a description of etter from Prof. Necker to Sir David Brewster, from I quote the following:-"You must conceive the r placed at the foot of a hill between him and the place he sun is rising, and thus entirely in the shade; the nargin of the mountain is covered with woods, or detrees and shrubs, which are projected as dark objects on right and clear sky, except at the very place where the ust going to rise; for there all the trees and shrubs borthe margin are of a pure and brilliant white, appearing ely bright and luminous, although projected on a most t and luminous sky. You would fancy you saw these ade of the purest silver." Necker says that he saw it at the Saleve, which is not above the Lake of Geneva as some of our British ins above the sea, and has no permanent snow near it ; M. Folie's suggestion, that it is due to light reflected ow, must be wrong. I have seen it from the König-See, hich I believe there is no permanent snow. appearance is always to be seen under the circumstances ed, when the sky is clear and bright enough. I had it in Tyndall's book, and when in the Alps I sought for nd it. I have often seen a distant approach to it proy furze bushes, quite near, seen against sunlight, and by gainst moonlight. JOSEPH JOHN MURPHY. --Ruskin somewhere describes this phenomenon. st, February 6. Foraminifer or Sponge? PER by A. Goës "On a peculiar type of Arenaceous ifer from the American tropical Pacific, Neusina Agassizi," been published in the "Bulletin of the Museum of Comp. at Harvard College," vol. xxiii. No. 5, in which the author s some remarkable forms dredged by the Albatross exin the Pacific of Central America. They are supposed to minifera, are of leaf-like shape, measure up to 190 mm. th, and are marked by concentric lines of growth. Their shows a stroma, consisting of fine chitinous threads, g sand and débris of shells. Without wishing to ree all the various points of structure, I will only say that n be no doubt that these forms belong to Hæckel's deep cosa (see Challenger report, vol. xxxii.) from the tropical and I should think that Neusina Agassizi is identical nnophyllum zonarium, Hæckel. I happen to have here nger specimen of this latter species, kindly lent to me Manchester Museum, and its microscopic examination s me of the identity of the two forms. rsity College, Liverpool. R. HANITSCH. Unusual Origin of Arteries in the Rabbit. TOWARDS the close of last month Prof. W. N. Parker reported 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, right subclavian artery arises beside the left subclavian, which at the point usually occupied by the innominate artery, while the occupies the usual position. PHILIP J. WHITE. University College of North Wales, February 7. Holmes's Comet. ON February 11, Ioh. to 10h. 35m., I re-observed this object with powers of 40 and 60 on my newly-silvered 10-inch reflector. The comet was in the same field as 8 Trianguli and south preceding that star. I found it fairly conspicuous. The nucleus, or brighter portion of 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 nebulosity, 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 excessively faint fan-shaped tail, but could not be absolutely The sky was not good, being lighter than usual, with suffused mist. On February 12, at 10h. 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. certain. W. F. DENNING. It This much-talked-of theory is a modification of Young's theory, which, by the choice of other fundamental sensations, endeavours to give better explanations of what it regards as immediate facts of internal observation. assumes three elementary sensations, related to three different parts of the nerve-apparatus or "visual substance." 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 66 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, corresponding to red, y to green, and Σ to violet, Hering's coordinates u, v, w will have the called dissimilation, and the latter assimilation. N u denoting the white element, and being measured along the axis of the pyramid; w denoting the red-green element, and being measured at right angles to the axis of white, in the plane containing the green edge of the pyramid; denoting the yellow-blue element, and being measured at right angles to the plane of u, w. Positive values of correspond to purple red, and negative values to green. Positive values of correspond to yellow, and negative values to blue. I give these equations in this definite shape for the purpose of showing, by a definite system of representation, that the arbitrariness which attends the choice of three colours, in terms of which the rest are to be specified, affords sufficient latitude to admit of the employment of three such different specifying elements as are adopted by Hering. If only positive values of x, y, z are to be admissible, the expression for shows that every kind of light must excite the white sensation positively, and consequently that no kind of objective light can produce a pure sensation either of the red-green or of the yellow-blue kind. Hence the pure unmixed "opposed colour-sensations" are such as we never have had or can have, and are separated from all colour-sensations that we have ever had by a much wider gap than the pure sensations which Young's theory supposes, although these latter extend somewhat beyond the range of objective colours. By subjecting portions of the retina to special influences (as we shall explain in treating of after-images) we can at least approximate to Young's elementary sensations; while these same methods, when we attempt to approximate to Hering's pure sensations, give results opposite to what his theory would lead us to expect. Hering assumes, in accordance with the brief expression of his theory in the above equations, that white light excites only the white-black visual substance and excites it always positively; that yellow light, besides doing this, excites the blue-yellow visual substance, as does also blue light, but in opposite sense. On the other hand, when blue and yellow lights are in exact equilibrium, they have no action on the blue-yellow visual substance. Similar remarks apply to the excitements of the red-green visual substance by red and green light. The sensation of luminosity is identified by Hering with the sensation of white. He accordingly maintains that the pure sensation of blue or of yellow involves no sensation of luminosity. I must confess that personally I can form no conception of a colour which has no degree of less or greater luminosity, and therefore think such an abstraction not tolerable in a system which, on other points, makes its appeal to the immediate testimony of inner consciousness, and claims by this means to establish its superiority to other systems. Differences of intensity must, however, occur in the opposed colour-sensations if they involve no difference of brightness. In comparing saturated blue with equally luminous pale blue, Hering would regard the white sensation as equally intense in both, but the blue sensation as stronger in the saturated blue. As the physiological basis of the "opposed colour-sensations Hering takes the two opposite processes of organic change, namely, the decomposition of the organic mass by activity, and its restoration under the influence of the circulation of the blood, which carries oxygen stored up in it and feebly united with it. The former process is This was a point which Hering left doubtful in the earlier statements of his system, so that it was not clear whether he assumed three or six independent variables. According to his more recent explanations the statement given in the text may fairly be said to represent his view. of the two opposed sensations corresponds to dissi tion and which to assimilation is left undecided, b the case of blue-yellow and of red-green. The logical improbabilities of this assumption have been pointed out already, and we shall retur: subject in treating of after-images. This assumption of double nerve-working was orizm applied by Hering to the white-black visual substance At the present time he adheres to the hitherto-t doctrines of nerve-physiology to the extent of t that, in the case of this substance, all light excites dissimilation and the sensation of white; and other hand want of light produces only assimat restoration of excitability. That during this later cess a sensation of darkness is experienced. 20 agreed. The difference is purely theoretical, Acr to the older view, which I have defended, we m order to perceive that there is luminosity in a pur part of the field of view at a given time, be able tinguish at another time that this perception is This perception that a sensation which might be not there contains in itself a testimony as to the c of the organ at the time, which is different fr sensations of incident light; and in this sense we . also a sensation-the sensation of darkness. Hering, on the contrary, maintains that the sens of black must have its own special physiological excitation, and seeks it in assimilation, going on white-black visual substance. From the foregoing account the reader will gate Hering's theory, if we overlook its physiologicalve able to explain all hitherto established facts mixture as well as, but not better than, Young's the It differs only in its special choice of elementary e tions; and this choice, if we admit negative them, suffices for expressing the facts, just as of co-ordinates suffice for a problem of solid geom Hering's objections to Young's theory rede selves, in his latest statement, to the following: "In the Young-Helmholtz theory, the assum the three elementary colour-sensations is à p sive, because these sensations are not presentat notoriously, according to necessity, now one se another set of elementary colour-sensations are a As to this, I have already remarked that the mental sensations of Young's theory, in so far. differ from objective colours, can be approximati the method of partial fatigue of the retina, mut closely than Hering's pure opposed-colour-sensa different upholders of Young's theory have made assumptions as to the three primary colours, an assigned different weights to various facts which the distinction, this affords no justification what the imputation that they have changed their assur according to necessity. It is always better 204 ledge existing doubt than to dogmatise. Hering goes on, "If the excitations belongs three elements have correspondingly distinct gical causes, one would expect that these s would have something special about them." This they have, in my opinion, in the promin of colour-saturation; for which, again, the opposed-colours furnishes no basis of explanati~ He continues, "Yellow gives, for example, the impression of a simple or elementary sensat violet, and yet we are told that the latter is an ec sensation and the former a mixture of simultane sations of red and green, or at least, in some product of the simultaneous existence of the excitations corresponding to these two e sensations." What a deceitful test apparent inner con in such matters, we can see from the examples authorities as Goethe and Brewster, both of whom eved that they saw in green the blue and yellow, of th, being misled by experience with pigments, they ved it to be composed. e goes on," Helmholtz says, quite correctly, 'so far as e, no way has been found of determining one of lementary colours except the investigation of colour Iness. This investigation has notoriously not coned Young's theory." his would, even if it were true, be in itself no argutagainst the admissibility of the theory. The theory olour-blindness seems, as we shall shortly see, to be rticularly hard crux for Hering's theory; while the rto well-established facts of red-blindness and greeniness admit of comparatively easy and perfect ination by Young's theory. mentioned that the law of superposability ceases to be applicable when the luminosity is excessive. Nevertheless, in view of the fact that simple colours of high luminosity are always as saturated as colours of such luminosity can be, it is not necessary, or rather it is not correct, to designate them as less saturated. The true statement is that differences of tint become more uncertain at high intensity-an uncertainty, which attaches also to the estimation of the intensity itself, as has long been known. If Hering's sensation of white and opposed-coloursensations are truly to deserve the name of elements or constituent parts of sensation (as he plainly intends, since he assigns to them special visual-substances), either he must acknowledge them as the elements deducible from the law of addition, or else they are purely hypo ▸ adds, “And the three sets of fibres, which, how-thetical processes of whose existence and superposability as Helmholtz remarks, are not essential to the y, have hitherto been sought for in vain." is objection applies to Hering's theory as much as Jung's. e reader will easily convince himself that these tions are of no weight whatever. He follows them an enumeration of contradictions and inaccuracies he professes to have found in Grassmann's and my explanation of Newton's law of colour-mixture, and y also in that of Kries, errors which, even if they ed, would in no way tell against Young's theory, but against its interpreters. Here, however, the Irity seems to me to lie on the side of our nent. ese objections arise out of the fact that, in mixtures saturated colour with white, the tint of the mixture times seems changed (pale red for example aches more to rose, and pale blue to violet); and on the other hand, with increase of intensity, the rs of the spectrum appear sometimes paler, someyellower. But if we speak of those elementary tions which, from the point of view of Newton's are alone entitled with certainty to the name of ents, as being able to coexist without mutual disnce, then the only sensation which can with cerbe regarded as corresponding to the coexistence of te and a red elementary sensation is that which s into existence under the simultaneous influence of orresponding white and red lights. The term "eleary sensation" is in this connection to be taken, of e, not in the narrow sense of Young's hypothesis, n the wider sense above explained-the sense in 1 we speak of linear relations between colourtions and linear superposition of elementarytions. In the domain of colour-mixture we know ng of any elements but these superposable ones; f we would preserve a constant meaning for our r-equations we must interpret them in this sense, have explained above. This is what H. Grassmann yself have always done. reover, erroneous estimates of the difference bea pale and a more saturated colour are liable to be , and hence those colours which are really most d with white do not always appear the palest. If, ut sufficient experience of colour-mixture, we only our judgments by similarity of sensations, we are liable ke mistakes as to which colour contains white. The on of the power of perceiving differences will therearise. Further, it is found that colours of very g luminosity do not differ so much from one another e sensations they produce as colours of moderate osity, a fact which finds its explanation in g's theory, of which it is a natural consequence. rs when highly luminous appear more similar ne another and more similar to white. We ss this by calling them pale as compared with s of feebler luminosity. I have, however, already no one knows anything. His polemic against Grassmann and me then amounts to this-that at a time when his hypothesis had not been propounded we did not speak in the sense of it. Hering seems to regard as the chief point of superiority of his own hypothesis its closer conformity with the names which have established themselves in language— names which, as I have explained above, relate rather to the colours of material bodies than to the colours of light. To this circumstance it is, in fact, indebted for a certain amount of popularity and facility of apprehension. He himself assumes that these names have sprung from an immediate perception of the simple elements of sensation by a kind of inner consciousness, and thinks that he has thus very certain and immediate knowledge of the pure red-sensation, the pure white-sensation, and so on. In his publication of 1887 he has discussed the possibility of assuming, instead of three or six simple processes of sensation, a larger and perhaps indefinitely great number, and a corresponding number of "elementary powers" for the several kinds of objective light. He, however, gives the geometrical representations of such actions in such a manner that practically these powers all depend on three independent variables. On the other hand, as regards these independent variables, which are the most important factors in the problem, he gives as good as no clue to them; he only seeks to remove them as far as possible from the sphere of physiology. my own part I am able to understand this whole series of descriptions only as meaning that an arbitrary number of visual substances can be assumed to exist in the brain, and that their respective strengths of excitation are different functions of the same three independent variables, each visual substance being unaffected by the excitations of the rest, and the excitation of each being susceptible of direct apprehension in consciousness. I do not think it is necessary, in this book, to go further into such hypothetical views. For Hering especially claims the credit of opening up the way to understanding colour-blindness. He makes all dichromasy depend upon a single cause, namely want of sensibility in the red-green visual-substance. The difference between red-blindness and green-blindness is, according to him, attributable to different colourations of the media of the eye; partly of the yellow spot of the retina, partly of the crystalline lens. These colourations are chiefly met with in the sick or the very old, and, when occurring in otherwise useful eyes, are not of such strength that they could bring out conspicuous deficiency of brightness in different parts of the spectrum. The colouration of the yellow spot of the retina takes effect in a very limited but very important part of the field of view, and in only a narrow band of the spectrum. The most trustworthy observations on the influence of the wave-length of the incident light upon the strength of the red and green excitations, have been made with kinds of light not liable to be absorbed in notable degree by the yellow pigment. On the whole, it is accordingly found that this pigmentation is subjectively influential only in cases in which the rays in the neighbourhood of the line F play a prominent part, as, for example, in a certain mixture of this blue with red (mentioned on page 354) which, if it looks white when our eyes are directly fixed upon it, will show blue predominant when we look in a slightly different direction. As far as hitherto-known facts go, it appears very improbable that Hering's theory of dichromasy can be carried through. Nevertheless, further observations in this direction are very desirable. The influence which the colouration of the yellow spot has in individual eyes can be estimated by comparing the appearances of colourmixtures in the centre of the field of view with their appearances very near the centre. Such comparisons will show with certainty where such influence is present and where it is absent. The following is a summary, by Prof. Everett, of two passages from the new edition of Helmholtz's "Physiological Optics," which are important as supplementing the foregoing critique of Hering's theory: In discussing the results of experiments for determining the exact positions of the three elementary sensations with respect to actual colours, in Newton's diagram or in Lambert's pyramid, Helmholtz represents the results by a triangle with the three elementary sensations at its corners, and with the colours of the spectrum plotted along a curve which lies entirely in the central portion of the triangle. He says, p. 457: "This curve shows that every simple colour excites simultaneously in the trichromic eye the three nerveelements which are sensitive to light, and excites them with only moderate differences of intensity. If we then hypothetically refer all these excitations to the presence of three photo-chemically alterable substances in the retina, we must conclude that all three of these must have nearly the same limits of sensibility to light, and must show, in the rates of their photo-chemical actions for the different wave-lengths, only secondary variations of moderate amount. Similar variations, arising from the presence of foreign substances, from substitutions of analogous atom-groups, and so on, occur also in other photo-chemically alterable substances as used in photography; for example, in the different haloid salts of silver." In a mathematical discussion of colour-blindness, commencing at p. 458, he points out that in dichromic vision there must be a linear relation between the three independent elements of trichromic vision, and in Lambert's colour pyramid there must be a certain line through the vertex, such that any plane drawn through it is a plane of uniform colour. Newton's diagram of colour may be regarded as contained in any plane which cuts the axis of the pyramid; and it is very important to determine the point in which the above-mentioned line cuts such a plane; for any line in Newton's diagram that passes through this point is a line of uniform colour to the dichromic vision in question. Experiment shows that it always lies outside the triangle of actual presentable colours. Addendum. Prof. Everett adds the following remarks of his own on the present position of the problem of colour vision:On the one hand, it is established, as a fact of experiment, that the excitation of colour-sensation in the normal eye depends upon only three variables, and that their effects are superposable, so as to admit of being expressed by equations of the first degree, otherwise called linear equations. The simplest choice of three variables is that adopted in Young's theory, because it only requires posi tive values of the variables. On the other hand, the various colours regarde subjective appearances do not naturally class them under a threefold heading. Yellow does not look it consisted of red and green. Colour-sensation known to us in consciousness are not threefol manifold. The two facts taken together seem to imply two se sive operations intervening between the inciders light and the perception of colour. The first open. threefold, and may consist (as above suggested by b holtz) of the photo-chemical decomposition of t different substances. The second operation comes the effects of the first operation upon a complex orga and the distinctions of colours as we see them as of the nature of this organism. The number of independent variables require specifying the condition of a system is a very de thing from the number of well-distinguished sa which the system can exist. For example, the s a given mass of water-substance is completely deter if its volume and temperature are given, and the depends on only two variables. But the number. well-distinguished states is three. In like manter depends on three variables, but the number of w tinguished colours, besides white, may be said to be namely the six principal colours of the spectr purple. What differences of condition in the organ respond to these eight distinct appearances in the view, and how these different conditions are produ the three primary excitations, are problems F solution. AUTOMATIC MERCURIAL AIR-PU F late years, and more especially during decade, men of science have devoted much and ceaseless energy to the invention of an a which should admit of the automatic working curial air-pumps. Of the numerous inventions forward, the ingenious apparatus of Schuller it are especially deserving of mention. But notwithstanding the present extensive emp of the mercurial air-pump in science as well as nics these appliances are neither much known, they been used to any great extent, although the great importance, and would probably be very adous. This may be explained by the fact that wanting in the necessary simplicity and trustw without which the advantages of automatically mercurial air-pumps are somewhat doubtful We shall describe now an apparatus for t fectly trustworthy and automatic working o air-pumps, as well as the shape of the glass p in connection with it, which, while possessing the possible simplicity, admits of the highest rat hitherto known. The figure shows the automatic apparatus in c with an improved Toepler mercurial air-pump. ball H is connected on the one hand by flew with the pump Q, on the other hand by the the accumulator M. The water-pipe K rus bottom of the accumulator, and by means of a** constructed three-way cock K can either be with the hydrostatic pressure-pipe K, or the c pipe K2 If water under pressure is admitted through k1, K and k into M, the air contained in M is at This air again exerts a pressure through the the mercury contained in H, and drives it in: Q. As soon as the mercury has risen sufficient the cock K is reversed, the compressed air water out again through A, K, and k, and t down on account of its own weight out of the PQ back into the ball H. ne reversing of the three-way cock, and therethe automatic action of the pump, is effected in ollowing manner:-The ball H rests on a frame D, ving about the axis b, and the motion of which is ed by the ledges c and c. A lever G is attached e frame not far from the axis, and by means of a when the balance D reverses its position, also turns ock. When the ball H is entirely filled with mercury alance D rests on the upper ledge c. If the pump is motion the left side of the balance D becomes r in proportion to the amount of mercury forced the ball H into the pump, until at last the weight C right-hand side becomes heavier, and the balance by attains the position shown by the figure. The way cock is also reversed by this motion. the adjusting of the height of the mercury can be easily and accurately done up to a centimetre. It goes without saying that every mercurial air-pump not provided with cocks can be worked by the apparatus just described. But the improved construction of the Toepler pump, drawn likewise in projection in the figure, has proved to be especially practical. The following is a description of its automatic working : If the cock t1 is connected with an hydrostatic air-pump, the ball Q of the pump and the space R, which is to be evacuated through the tube S, is pumped out up to the tension of the vapour. The mercury then rises in the tube R almost to the height of the barometer above its level in the ball H. If the automatic apparatus is then set in motion, the mercury enters the ball Q and the tube s, thus cutting off the connection with R, while any further rising of the mercury as already described, the water current is now the water present in M flows out through K2, and cury goes back from the pump Q into the ball H. g the tipping over of the balance, however, the weight C has run down its inclined plane to a so that it now exerts a pressure on the lever momentum is so calculated that the mercury in p must have fallen to the point p, and flowed back ball H before it again overweighted, and moves balance. The weight C then slides back again ft until it rests against its left ledge, and the play ump recommences. It will easily be seen that ht to which the mercury rises in the pump, the the sliding weight being a constant quantity, only on its final positions, and that, therefore, in the tube s is prevented by a glass valve v, it passes through the first V-tube r, filling the little vessel r2 and rises through s1 into the ball n, driving before it the air which was before shut off in Q. At this moment so much mercury has been forced out of the ball H into the pump Q, that the balance is turned, the mercury flows back out of Q into H, forming vacua in r and Q, as the little mercury-threads remaining in the sidetubes, and s, form shut-off valves. As soon as the mercury has fallen below the entrance-point of s into E the pressure in R and Q become equal, the denser air flowing out through S into Q. The time during which Q is connected with R may be determined at will by changing the right ledge of the sliding weight. Then the balance again changes its position, the mercury rises in |