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central Africa; and it was likewise the home of an extensive and varied fauna of game and other edible birds. To this abundance of wild life is attributable the comparative facility with which the country was explored and settled; but no sooner was the settlement well advanced than ruthless slaughter led to the more or less complete extermination of some species, like the bison and the carrier-pigeon, and a vast reduction in the numbers of others, such as the prongbuck and bighorn. Fashion, sport, and other factors. led, later on, to equal havoc among birds of many kinds.

Having permitted all this to come to pass, the country is now gradually waking up to the loss. it has sustained, and to the remedial measures still possible in order to ensure the preservation of at least a remnant of the ancient superfluity of life. In this crusade Dr. Hornaday has for many years been a leader, and in the volume now before us he reviews what has been done and what still remains to be accomplished in a manner worthy of all commendation.

So urgent, however, is the case that the author calls upon all biological workers to abandon such comparatively unimportant matters as the description of species and races, the protective coloration of eggs, etc., and to devote all their energies to the cause of protecting and re-habilitating their country's fauna, not only as a foodsupply, but, in the case of insectivorous birds, as a protection against the ravages of insect-foes, which are specially severe in America.

In his opening chapter-on the value of wild life-Dr. Hornaday enunciates the axiom that no species of wild land animal can long withstand systematic hunting for commercial purposes, as witness the destruction of the millions of the southern bison-herd within four years. He also points out that when a species has become reduced below a certain number it loses all recuperative power, and, like the heath-hen, fails to respond to protection. Instant action is, therefore, imperative in order to save the present remnant of the game-fauna, which is estimated to be only 2 per cent. of its former numbers. For this purpose "bag"-limitations have proved practically useless; and the conversion of national forests into reserves where shooting shall be absolutely prohibited, is a sine qua non (p. 49).

The period from 1885 to 1900 saw the great boom in the plumage-trade, to check which the Audubon Societies were organised; other agencies, which in many cases are proving victorious, came into action soon after; but the greatest hope for ultimate salvation is the federal law of 1913 for the protection of migratory species,

which, by bringing recalcitrant States into line, saved the situation. Lastly came what it is hoped will prove the winning card in the shape of the Feather Bill. R. L.

FOUR DIMENSIONS. Geometry of Four Dimensions. By Prof. H. P. Manning. Pp. ix + 348. (New York: The Macmillan Co.; London: Macmillan and Co., Ltd., 1914.) Price 8s. 6d. net.

PERHAPS the main interest of this work is

that it treats the subject in a way that is comparatively new, and one that is likely enough to be generally adopted. Until quite recently, most works on hypergeometry might be roughly divided into three classes: popular or semipopular outlines, which, however stimulating or suggestive, have little or no scientific value; frankly analytical disquisitions, such as those of Riemann, etc.; and works which, although couched in geometrical language, give the impression of being, so to speak, translations of previous analytical demonstrations. It must be admitted, of course, that some authors (such as Segre) have obtained new and valuable results for surfaces in three dimensions by considering them as sections of hyper-surfaces, and have pursued other four-dimensional researches in a way which has much which has much more the aspect of being purely geometrical. But since it is a psychological fact that so far we have no true intuition of four-dimensional space, the inference seems to be that these authors have become so familiar with the analytical arguments underlying their theorems that they pass without an effort to the corresponding geometrical form of statement; much in the same way as dualisation of a projective theorem becomes almost mechanical after sufficient practice.

The method of the present work may be described as a logical induction based upon explicit geometrical axioms about strictly geometrical indefinables. The primary element is the point; the primary undefined relation is that of a point P being collinear with a given segment AB. From this the definition and properties of a straight line are deduced; thence we proceed to the definition of a triangle; and from this, with the help of Pasch's axiom (a line meeting one side of a triangle and another side produced meets the third side), we arrive at the definition and properties of the plane.

Assuming, now, that after reaching a plane field of points there is at least one point not belonging to that field, we can construct a tetrahedron, and, by arguments strictly analogous to those employed before, arrive at a three-dimen

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The important thing to notice here is that the

argintuition at all. It is true that by drawing figures, or making models, we can provide images which help us to see that for an S1, S2, or Sg our assumptions are not self-contradictory-or rather to give us an irrational conviction that things are so; for, of course, nothing but a formal proof can demonstrate the consistency or inconsistency of a set of formal propositions, such as we are ultimately dealing with here.

When the reader has reached this point, he will

see that we can define and investigate a sequence of spaces:

...

S1, S2, S3, . . . Sn, . . (each being a field of points) on the assumption that when we have reached a space S, there is at least one point which does not belong to it. A space S is determined by (n+1) independent points; namely, such as do not belong to one and the same Sn-1.

The author does not go further than the S4. After establishing its (logical) existence, he proceeds to discuss perpendicularity and angles; symmetry, order, and motion; hyper-surfaces and polyhedra; the theory of parallels; and that of volume-measurement. Finally, there is a chapter on the regular "polyhedroids" (hyperpolyhedra) in the S..

There are many interesting details and illustrations; we may refer to one of these, as it shows very well the way in which we are brought to a halt in trying to make actual images of things in the S4. If, in an S2, we draw a square, then a square on each of its sides, and finally a square on one of the outermost sides (so as to make a Latin cross) we can see how to fold the squares about common edges until they form the faces of a cube in our S. Suppose, now, that in our Sg we draw a cube; then a cube on each of its faces; and finally a cube on one of the outermost faces of the last. If we could get this solid into an S4, we could "fold" the eight cubes about adjacent faces so as to form the boundaries of a hypercube. Until we can "see" how to do this, we have no proper intuition of an S4, such as we have of the lower spaces. G. B. M.

THREE BOTANICAL BOOKS.

(1) Mikrographie des Holzes der auf Java vorkommenden Baumarten. Vierte Lieferung. By Dr. J. W. Moll and H. H. Janssonius. Pp. 336. (Leiden: E. J. Brill, 1914.)

(2) Practical Field Botany. By A. R. Horwood. Pp. xv+ 193. (London: C. Griffin and Co., Ltd., 1914.) Price 5s. net.

(3) All About Leaves. By the late F. G. Heath. Pp. ix228. (London: Williams and Norgate, 1914.) Price 4s. 6d. net.

(1) IN this, the fourth instalment of his woof Javan trees, the author deals with the families Connaraceæ, Leguminosa, Rosacea, Saxifrageæ, Hamamelideæ, and Rhizophoreæ. The Leguminosæ occupy more than half the volume.

on the intimate structure of the

In this

family 118 specimens were examined, representing forty-nine species and four varieties contained in twenty-six genera. Under the heading of each family is a list of the literature in chronological

order, followed by an enumeration of the species. and varieties examined by the author. A general review of the gross and minute anatomy of the family as represented by the forms studied is then given, and the bearing of the results on the generally accepted sub-division of the family is discussed. These are, in the main, in conformity with the sub-division based on floral structure, but it is worthy of note that in Rosacea the differences between the groups are greater than is usually the case in a single family, and suggest the recognition as distinct families of the Amygdalaceæ, Chrysobalanaceæ, and Pomaceæ. A key for determining the species by means of the wood-structure is also given. Then follows a detailed description of each species or variety; first the general topography, which is sometimes illustrated by figures, and then an elaborate description of the various elements-wood-vessels, libriform fibres, wood-parenchyma, medullary rays, etc.

The account includes measurements

and the behaviour of the various elements to reagents. It is to be regretted that the author did not include photomicrographic reproductions of his sections; the few figures which are given are poor and show no detail.

(2) The title of Mr. Horwood's book is somewhat misleading. The book contains much useful information on the practical study of plants in the field, but also much extraneous matter often set down in a loose and desultory manner. The impression formed by the reader is that the author is a man of great industry and some knowledge, but does not realise his limitations. The book, if judiciously pruned and edited, would make a useful little volume. The author's intention is

"to explain and set forth the principles by which the ecologist should be guided, and the apparatus. and plan to be adopted to enable him or her But to work upon sound and approved lines." the directions or suggestions will often lead to bewilderment; for instance, the section on pp. 4-6, entitled "What to Study," contains a curious medley of suggestions from the study of the form and size of pollen-grains to the procuring of "fruits and berries of foliage" (sic) for decoration at Christmas. On the other hand, the same chapter contains a useful section on flower-photography. Chapter ii., on methods of collecting, preserving, mounting, and storing plants for herbaria, contains much that is helpful, though the student will find puzzling matter in the section on "the arrangement to be adopted in the herbarium"; among other things the author suggests the preparation of "a printed list of all the species published since the year 1895 when the Index Kewensis' and Durand's Appendix' brought things up to date," thus ignoring the three supplements of the "Index Kewensis" which carry us on to the end of 1910.

A great part of chapter iii., which has the tremendous title, "The necessity for encouraging the study of botany on ecological lines, by the popularisation of pure life-histories of plants, through nature study, museums, scientific societies, and other associations, and in the university," might well have been omitted; much of it consists of extracts from the addresses or writings of well-known botanists. The next chapter, on the study of the life-history of a plant, would have been more helpful if in the brief paragraphs on the various divisions, such, for instance, those on "plants and fungi and insect-pests"

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names of wild plants" reference had been given to a good standard work on the subject. The last chapter (v.), "An outline of plant formations," is the best; it includes a brief description of various plant-formations and lists of the species which occur in them.

(3) The little book on "leaves" contains eighty photographs, mostly of British wild plants, which on the whole are good. They were specially prepared for the work, but were not seen by the author owing to his death in 1913. The text is in the style with which readers of the late Francis George Heath's botanical works are familiar. One example must suffice.

Of the seed we read, "A created organism of wonderful and infinite skill confronts our marvelling sense in the remotest confines of the great vegetable worldso that we cannot begin at the beginning because we cannot comprehend the beginning! Powerunquestionable-hovers, so to speak, undiscernible in the tiny seed."

OUR BOOKSHELF.

The Golden Bough. A Study in Magic and Vol. xii. Religion. By Sir J. G. Frazer. Bibliography and General Index. Pp. vii+536. (London: Macmillan and Co., Ltd., 1915.) Price 20s. net.

No book has ever been written which contains so large a mass of facts as "The Golden Bough." Nor has any book had its data more thoroughly documented. The bibliography and index, forming the twelfth and final volume of the work, fill 536 octavo pages. The index, 392 pages, is fuller than the indices to the separate volumes. Every author knows the labour of cross-references; a simple instance here is "propitiation of vermin by farmers," involving three entries. There are cases of overdoing, a good fault in an index, e.g. “Nat, spirit, in Burma, ii. 46." "Nat superstition in Burma, ix. 90 n1." "Nats, spirits in Burma, iii. 90; ix. 175 sq.; propitiation of, ix. 96." Here a distinction is actually drawn between a singular and a plural. Technical generic terms in foreign languages are, like botanical and zoological terms, etc., printed in italics. But why "Oschophoria, yet "Aiora," both Greek feasts; and "Farwardajan," yet "Sada," both Persian feasts; also "Ogboni" and "Belli-Paaro," and "Belli-Paaro," African secret societies, yet "Ndembo" and "Hametzes," also secret societies? But these are minor inconsistencies in a monumental index.

The bibliography comprises probably six thousand books, including serial publications and dictionaries. The curious may find instructive items, e.g. Maeletius (Maletius, Meletius, Menecius, Ian Malecki), who, by the way, wrote on the religion of the Borussians (the present-day Prussians). Many periodicals, e.g. L'Année Sociologique and L'Anthropologie, have not their year of institution, nor, what is far more important, their place of publication attached. How are "readers who desire to have further information," to find "Fasciculi Malagenses, Anthropology," or "Dinkard, a Pahlavi work "?

The distribution of anthropological research among the civilised nations may be well estimated from this bibliography. England, America, and Holland are prominent; Germany has done much, so has France. The native authorities on uncivilised peoples are an interesting addition. A. E. CRAWLEY. The Plateau Peoples of South America. By A. A. Adams. Pp. 134. (London: G. Routledge and Sons, Ltd., 1915.) Price 3s. 6d. net. THE people discussed in this monograph are those inhabiting the South American plateau within the boundaries of Bolivia, Peru, and Ecuador. Bolivia, however, seems to be more closely associated with the past history of the plateau than either of the other republics. Its government is largely drawn from the plateau population, and its general culture is more clearly influenced by the plateau than that of Peru, which looks to Europe for light and guidance. The writer finds in this upland race a condition of progressive degeneration. The people who occupied the country in what may be

called prehistoric times were skilled in stoneworking, as is shown by the great megalithic ruins at Tiahuanaca, described by Sir Clements Markham in his book, "The Incas of Peru." Since the building of this city, geological changes seem to have been in action which caused the elevation of the plateau and the shrinkage of the body of water now known as Lake Titicaca.

These changes brought about the present conditions a very dry atmosphere, with a small percentage of oxygen and a high range of temperature, inducing nervousness and mental instability, a lack of forethought and industry, an weening contempt of foreigners, and a perfervid patriotism. Many physiological facts indicate this retrogression, resulting in administrative incapacity, and neglect of regularised education. The poorness of the food supply, ill-cooked potatoes and maize-flour cakes, promotes physical degeneration and leads to over-indulgence in stimulants. The writer takes, perhaps, too gloomy a view of a people whom he dislikes and despises, but he appears to write with adequate knowledge, and his monograph, if his conclusions be accepted, furnishes a good example of the action of an unfavourable environment upon a race exposed to its influence.

A Pilgrim's Scrip. By R. Campbell Thompson. Pp. xii + 345. (London: John Lane, 1914.)

Price 12s. 6d. net. THESE slight, discursive sketches of the life of a wandering archæologist in the Nearer East are interesting and instructive. The studied archaism of the style, a trick which may have been learned from Mr. Doughty's famous "Travels in Arabia Deserta," becomes, after a time, a little monotonous, but it gives a piquant flavour to his accounts of eastern life and character.

The writer

is one of the school of scholarly antiquaries, trained by the British Museum, who, in spite of many hardships and the necessity of making scanty funds go a very long way, have done noble service in adding to our national collections.

Mr. Campbell Thompson's experience has given. him a considerable insight into the back of the oriental mind, and his hints for dealing with these races and conducting excavations will be serviceable to those who may follow his tracks. His wanderings have extended widely: Mosul, Behistun, the Sinaitic Peninsula, the Sudan, Angora, and Carchemish, are some of the stages. Perhaps the most interesting episode is his excursion, in company with Mr. L. W. King, to make fresh copy of the famous inscription of Darius at Behistun, the riddle of which was solved by the genius of Sir H. Rawlinson. Swung from cables suspended over the precipice the explorers collated Rawlinson's copies, which proved to be wonderfully accurate, and succeeded in photographing, from a five-foot range, the splendid head of the warrior king-a fine piece of work told modestly and clearly.

The book is well illustrated by photographs and, which is unusual in popular works of travel, is provided with an excellent index.

Morale Fondée sur les Lois de la Nature. By M. Deshumbert. Cinquième et Sixième Mille. Pp. 191. (London: Watts and Co., n.d.) THE Comité International de Propagande pour la Pratique de la Morale fondée sur les Lois de la Nature has representatives in eighteen countries. Its Bureau Central is at Dewhurst, Dunheved Road West, Thornton Heath, England. It issues. a propagandist volume on the subject of a natural morality, written by the secretary, M. Deshumbert, which has been translated into eight languages. Much has been written on morality "according to nature," since the Stoics invented the idea, but this book, partly because the author understands both physiology and biology, has a freshness of appeal."Good is all that contributes to the conservation and increase of life . . ." by co-operation and mutual aid of individuals each of whom is thus aided towards complete self-realisation. Evil is all that diminishes life. The end of Nature is life and more life.

These and connected axioms are well illustrated by examples of anti-natural human superstition and of the importance in the animal world of intellectual and moral qualities. The way in which, e.g. the tiger depends for existence upon observation, judgment, patience, self-control, decision, and perseverance, is quite a fresh objectlesson. A collection of practical rules of personal hygiene and a detailed list of physiological functions are useful, and might form the nucleus of a modern scheme of individual morality. Some quotations from J. Payot are interesting here. A set of parallels between the intelligence of nature and of man is interesting, and might be augmented. "Man in many cases is inferior to nature," but this simply points the truth that man is part of nature.

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 communications.]

Ultra-Violet Excitation of the D Line of Sodium.

It is well known that the D line of sodium is the first member of a series of lines, the other members

being in the ultra-violet region of the spectrum. It is known also from the investigations of R. W. Wood that sodium vapour at a moderate temperature illuminated by D light gives rise to a secondary emission of D light. This secondary emission is appropriately called by him resonance radiation. It has been further investigated by Dunoyer.

The first ultra-violet line of the series is situated at wave-length 3303. It is very probable for a number of reasons that this or any other line of the series would give rise to resonance radiation, though I do not know of any experiment directly establishing the fact.

A more doubtful question is whether stimulation by the line 3303 would give rise to D light. The question is not new. It has been proposed by Prof. Wood, and he has looked for the effect, but without success. The matter seemed important enough, how

ever, to justify a fresh effort, and within the last few days I have been able to get the effect as satisfactorily as could be wished.

The chief essential is a source giving a sharp line of great intensity at A3303. This was found in a sodium vapour lamp in quartz analogous to the mercury lamps in general use. Details of the construction and manipulation of these sodium lamps will be published later. The visible light from such a lamp was filtered out by means of a screen consisting of cobalt-blue uviol glass, combined with nitrosodimethylaniline. The fight which came through was photographed with a quartz spectrograph, and was found to consist of A 3303 exclusively.

This radiation was concentrated by means of a quartz lens on a quartz bulb containing some sodium. The bulb was made nearly red-hot with a bunsen burner, which was then extinguished. A patch of luminosity could be seen on the wall of the bulb when the ultra-violet beam fell upon it. As the bulb cooled and the vapour pressure of the sodium diminished, this patch of light gradually expanded, and filled the entire bulb; it then faded away, and had disappeared when the bulb was cold. This behaviour is exactly the same as is seen when D light is excited by the incidence of D light, and although in the present case the light is much fainter, the conditions of observation are in some respects more favourable, for there is no disturbance from visible light scattered or reflected by the walls of the vessel.

Critics of this experiment will naturally concentrate their attention on two questions :

(1) Was the light observed really due to ultra-violet excitation?

(2) Was it of the same wave-length as the D line? As regards (1), a sheet of plate-glass 1-2 cm. thick was interposed between the source and the bulb. The excited light was completely extinguished.

As regards (2) the light was rather below the intensity which would easily allow of direct spectroscopic examination, though with a little further improvement of the conditions it might be made strong enough. I have, however, proved it to be of approximately this wave-length by absorption methods. The luminosity was seen undiminished through a thick cell containing potassium bichromate solution, held before the eyes. It was absolutely invisible through a cell containing praseodymium nitrate. Thus the wavelength must lie in the region from A 5820 to A 6020, for this is the only region transmitted by bichromate and absorbed by praseodymium. The D line at A 5890 lies in this narrow region, and I think, therefore, that there is no reasonable doubt that the emission does consist of D light. Discussion of the theoretical bearing of this result is deferred.

R. J. STRUTT. Imperial College, South Kensington, May 8.

The Green Flash.

MANY descriptions of the green flash have been published in letters to NATURE and elsewhere, but I do not remember to have seen a satisfactory explanation of this curious phenomenon. Atmospheric dispersion is invoked, but this does not explain the absence of the red end of the spectrum. My observations agree in every particular with those described by Mr. Whitmell in NATURE of March 11, p. 35. At sea I have observed a violet or blue tint occasionally, and on one occasion a red flash as the lower limb of the sun emerged from a cloud into a clear space very near the horizon.

Normal atmospheric dispersion will, of course, produce a red fringe to the sun's lower limb, and a blue fringe at the upper limb, as may be seen at any

time with a telescope free from secondary colour when the sun is as high as ten or fifteen degrees above the horizon. When, however, a point of sunlight only is visible, the rest of the disc being hidden beneath the horizon, atmospheric dispersion, if it could be perceived with unaided vision, should produce a completevertical spectrum from blue to red, as in the case of stars when near the horizon. The red end of this. spectrum should be most evident, since these rays are least absorbed. In the flash, however, the red is completely suppressed, and the vivid green which is obvious to the naked eye can only be seen at very low horizons. Moreover, it is not always seen, as Mr. Whitmell remarks, when the conditions seem otherwise favourable.

It seems to me very probable that the phenomenon is in some way connected with the abnormal conditions which at sea produce mirage effects. The layer of dense air in contact with the sea might produce total reflection for solar rays refracted from below the horizon, but the critical angle of reflection will depend on wave-length, and it is possible under certain conditions that the green rays may be totally reflected whilst the red are refracted.

I have one more observation to add to those described by Mr. Whitmell, and this will, I think, give the coup de grâce to the theory of a subjective effect due to retinal fatigue. In May, 1900, I happened to observe the setting of Venus in the sea from my eclipse camp on the Algerian coast. Observing with a 3-in. inverting telescope, I saw the planet when very near the horizon suddenly change in colour from dull red to vivid green, and as I lowered the telescope to the point where the sea horizon about bisected the field of view I was amazed to see two green images of Venus, one, the normal image, ascending from below, and the other sloping down from above. This was probably reflected from the sea itself. The setting took place at the moment of meeting of these two images. The whole apparition, from the moment when the colour changed from red to green, to the instantaneous disappearance of the two images, cannot have lasted more than four or five seconds. The sea about this time was found to be excessively cold, although the air was hot during the daytime, and this state of things would doubtless favour the production of a relatively dense layer of air on the surface of the sea in calm weather. JOHN EVERSHED.

Kodaikanal, April 13.

The Larger lons in the Air.

In addition to the well-known small ions, which are of a type common to all gases, two classes of larger ions exist in the air under ordinary conditions. One of these consists of the large ions of Langevin which have a mobility of about 1/3000, while the other contains ions with a mobility of about 1/50. As the latter value lies between those of the mobilities of the small and large ions, the members of this latter class may be called the ions of intermediatemobility, or, shortly, the intermediate ions.

The slow movement of these larger ions in an electric field clearly indicates that they are molecular clusters of more or less complexity. Ordinarily the value of the mobility is the only guide to the nature of the ionic structure, but in the case of the large ion, at least, an important deduction is to be made from the outcome of experiments on the formation of clouds in closed vessels.

It is well known, since Aitken's notable work on the subject, that, in ordinary circumstances, the air is crowded with particles, in suspension, on which the water vapour condenses into visible drops if the air becomes slightly supersaturated. These particles,

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