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set free by extrusive agency, and continues to be a offers most of novelty, but the larger portion of the source of supply to the present time. The nature of two volumes deals, on a more familiar model, with the primitive atmosphere may be conjectured from the the several geological periods in order. The Archæan known occluded gases in crystalline rocks and era is regarded as representing the climax of igneous meteorites, having regard also to a certain selective action (or, as it is confusingly styled, volcanic actioni, effect depending on molecular weights. Carbon and as being concurrently a time of intense crustal dedioxide was probably abundant and nitrogen only a

formation. The Huronian and other pre-Cambrian minor constituent, the latter, in virtue of its chemical formations which follow the Archæan are grouped a: inertness, having accumulated progressively through- Proterozoic-an unfortunate choice, since the naine out subsequent time. It is supposed also that the has already been used by Lapworth for the Loner oxygen in the present atmosphere has mainly been Palæozoic. The Lake Superior region is taken as the set free by the agency of vegetable life. The typical area, and three distinct systems are recognised initiation of vulcanism is next considered, involving a --Huronian, Animikean, and Keweenawan. Thr discussion of the thermal conditions in the growing great fossiliferous systems are then dealt with in turn. globe. The heat produced by the infall of the planet- the chief innovations as regards systematic arrangeesimals was probably important only in the earlier ment being the division of the Carboniserous into two, stages of growth, and the chief source of the earth's Mississippian and Pennsylvanian, and the separation internal heat is ascribed to the progressive compression of the Lower Cretaceous as a distinct system under of the central parts. It is estimated that this cause the name Comanchean. Under each head the developalone would suffice to reach the melting temperature ment of the stratified sequence in the North American of rock when the earth had acquired one-tenth of its continent is described and its interpretation discussed, present mass. On account of the originally hetero- the probable geographical conditions of the North geneous composition of the globe, local spots of fusion American area at different periods being illustrated by would arise, the occluded gases presumably playing maps. The corresponding strata of other parts of the a part in the process, and, aided by the varying differ- world are dismissed more summarily. This plan is ential attractions of the sun and moon, the molten

natural in a work designed primarily for American matter would gradually work its way outward. This students, and its inconvenience is felt only in certain action is supposed to be facilitated by “selective cases where the American record is incomplete or in. fusion,” the more fusible materials encountered being adequate, especially in the Permian and Jurassic taken


and the more refractory of the old materials periods. We have, however, as a digression, a good deposited. In the general theory of igneous action account of the widespread glaciation in the southern developed by the authors there is evidently much that hemisphere in Permian times, with excellent figures is debatable. In particular, the assumption that

the assumption that (after Schwarz) of glaciated rock-surfaces and boulder minerals have their melting points raised without deposits in South Africa. We think that the authors limit by increased pressure, is one to which many have succeeded in giving a fairly complete and weilphysicists will demur. The maximum melting point proportioned sketch of the earth's history in its found by Damien and others for various organic successive chapters. The only serious defect which bodies, and considered by Tamman to be a general we find is the slight notice accorded to igneous action, property, has led Arrhenius to very different con- and especially the failure (except in the earliest clusions concerning the actual condition of the earth's chapters) to recognise this as an essential part of geointerior.

logical history, closely bound up with the lectunic Another part of our authors' system which fails development of the globe. to carry complete conviction is the explanation offered For reasons connected with the curriculum of for the initiation of the ocean-basins. The cardinal | American universities, the history of life is treated in fact to be accounted for is the lower density of the great measure apart from the physical history of the crust in the continental areas as compared with that earth, a plan not without practical disadvantages. beneath the ocean floor. The difference is here No attempt is made to give a complete " roll-call attributed to the weathering and leaching action on of the flora and fauna of each period, but attention is the land, as contrasted with the relative protection of directed especially to the main lines of biological de the rocks under the sea. It is supposed that the velopment from the evolutionary standpoint. As reselective action of degradation and transportation sets gards the evolution of life in general, it is supposed up in time an appreciable difference in composition that more than half of the complete history antedates between the average material of the continental and the first fair record, offered by the Cambrian strata, that of the suboceanic tracts, the former becoming in which we have abundant evidence of a development more acid and so lighter, and the latter more basic already far advanced. For this reason the Cambriin and therefore denser. The effect would be cumulative, faunas are dealt with at some length. Similarly, in and the difference of density established would be the Carboniferous we have for the first time a large permanent, not being obliterated by subsequent meta- mass of material bearing on the evolution of plant morphism. In this way there might be evolved, from life, and this receives due notice, with a digression an originally fortuitous disposition of the growing discussing the origin of coal and the climatic conhydrosphere, a distribution of land and water having ditions implied in the profusion of vegetable life at .a high degree of relative permanence.

that epoch. We have dwelt on that part of the work which The arrangement of the book is in most respects





to see.

well adapted to the requirements of students, and the seems to be feeding-bottles. We are inclined to think presentation of the subject-matter is always clear. In that too much stress can be laid on the existence and the biological sections Transatlantic freedom of style qualifications of the first class. A long series of inis sometimes carried so far as to savour of the evening ductive reasonings, followed generally by equally Press, paragraphs b:ing headed, for example, " New laborious experiment, is the usual course of a Devices of the Bryozoans” and “The Protozoans successful invention. Helmholtz and Darwin make a Record.” The abundant figures are well not inventors, but their methods were the chosen, and, within the limitations of black and white, Helmholtz said that in his work he could only liken usually well executed, but the glazed paper, on which himself to the mountaineer, painfully and slowly the whole is printed, is an offence to the sensitive climbing, often obliged to turn backwards, lighting eve. The work as a whole is one which will find a

later on

traces leading forward, and• finally welcome in England as well as in America. The reaching the goal, only to find to his confusion that planetesimal theory, too, whatever its ultimate fate, is a plain road led thither, if he had only had the eyes at least a spirited protest against any narrow limit

Darwin said he thought he was superior to ation of geological time, and may serve to fortify timid the common run of men in noticing things which geologists against the thunders of certain mathe- easily escape attention, and in observing them carematicians, too apt to forget the precarious basis upon fully. My industry has been nearly as great as it which their calculations are built.

A. H. could have been in the observation and collection of

facts." Herein lies the real spirit of the pioneer.

Nothing is more useful than the quality on which THE GENESIS OF THE INVENTOR.

Darwin naively lays stress, viz. that of noticing Erfindung und Erfinder. By A. du Bois-Reymond. things which escape attention; and those who hope to

Pp. vi+284. (Berlin : J. Springer, 1906.) Price reach the promised land without wandering in the 5 marks.

wilderness are probably doomed to disappointment. IN

his opening chapter, Herr du Bois-Reymond Superficially, chance seems to play a large part; but

gives an historical survey of the development of Herr du Bois-Reymond maintains that chance only the Patent Laws in civilised countries. They date determines whether this or that individual shall do irom the Act of Parliament passed in the year 1623, the deed, and has nothing to do with whether or not which in its first clause abolished the long-standing the deed shall be done. This is probably true in those grievance known as monopolies, by which favoured cases in which attention is directed to a problem from individuals had the exclusive right to sell such things various sides owing to a main directing cause. Such as salt and coal; the second clause established a was the result of Moissan's discovery of the producnew variety of monopoly, out of which patent rights tion of calcium carbide in the electric furnace. The had their origin. Little has been altered in principle acetylene generator seems to follow as a matter of since that date. Even down to the term of fourteen course. Moissan had no heed for the commercial years the system still holds good, rights being exploitation of such things, and many others, begranted to “any new manufactures.” Other coun- coming aware of the existence of an obvious need, tries, adopting the idea at much later dates, attempted which appeared to be capable of being dealt with a more formal definition of invention, and legal logic without the aid of the calculus, rushed in, left the has constantly tried to define the admissible and the relics of their labours in the files of the Patent Office, inadmissible. Herr du Bois-Reymond shows that in and discovered later that they were wholly unGermany, since the year 1889, the number of patents acquainted with the conditions of the problem. In granted has varied between 29 per cent, and 45 per this case mere inspiration leads nowhere; laborious cent. of the number of applications filed, and, there experiment is much more to the point, and chance fore, assuming the quality of the inventions to be on only comes in, having regard to the number of men an average the same from year to year, it would at work on the task, in determining who shall lodge seem that the official mind is not yet certain in its his application first. That cannot properly be called workings.

chance which is merely the outcome of some The author's analysis of the nature of invention looked-for combination or slight variation of proand inventors leads to the conclusion that neither cedure; it is precisely for these things that the need, nor chance, nor the lack of necessaries in inventor toils, and when they come within his sight surrounding life suffices to draw out the inventor. | he merely recognises that for which he has patiently Instead of solving the problem by philosophic deduc- hoped. tions from generalities, he descends to the particulars Herr du Bois-Reymond concludes by considering the of the Patent Office, and concludes that inventors reaction on civilised life which is due to the existence can be subdivided into three classes :—first, the of the inventor. The idea of protecting the inventor intuitive genius, or, as Herbert Spencer would have was only an indirect cause of the Patent Laws in said, the man who can do with little trouble that most countries. A more direct impulse was probably which cannot be done by the ordinary man with any given by the view that the prosperity of the State was amount of trouble; sccondly, the technical man, well likely to be increased by such encouragement as could acquainted with his work, who follows in the wake be given to the creation of industries. Still, Faraday's of the intuitive genius, and is largely inspired by commercial value has been incalculable, but he rehim; thirdly, the layman, whose special province ceived little encouragement from Patent Laws, while



Watt was obliged to circumvent them in order to The volume on Ethics has some excellences, the carry his business. Moreover, the State discussion of the origin of morality, for example, with doubtedly profits directly. It is asserted by men what the author regards as the most important procompetent to judge that the amount received in patent position he has to offer, viz. that organic evolution, fees is greater than all the profits made by in- reproductive evolution, and moral evolution are interventors. In other words, the average profit made on dependent. Some other things are not quite so conan invention is not sufficient to cover the charges vincing—the statement that there has been far more made by the State. Herr du Bois-Reymond's book vicious than virtuous obedience in human history, or may be recommended to those who take an interest | another that morality is æons of æons older than the in the philosophic analysis of these questions, and oldest creed, the proof offered being that a cat carts they may also hope to find much worldly wisdom for its kittens. Apparently morality began ages before scattered throughout its pages, and a wealth of illus- man was ever heard of, though, in a different context, tration, drawn from the experience of a busy life. Dr. Saleeby describes a baby as non-moral, pre

W. H. S. moral, or if you like, immoral.”

There is a hard saying on one page to the effect

that historians of the (inaccurate and picture-que BIOLOGICAL PHILOSOPHI'.

school of Carlyle and Froude are no longer in request. Psychology (pp. 124); Sociology (pp. 124); Ethics

This comes with rather a bad grace from one (pp. 118). By Dr. C. W. Saleeby. Three vols.

whose merits are probably-quanto intervallo!—much Scientific Series. (Edinburgh and London : T. C.

like those of the writers named; while his defects and E. C. Jack.) Price is. net each.

include an inadequate apprehension of the real issues DR. R. SALEEBY discusses the problems of philo- involved and a stumbling knowledge of Greek. For

sophy from the Spencerian standpoint in an logos does not mean science, nor is teleology derived interesting fashion. Of the three volumes, that on from the word meaning “at a distance." Psychology appears much the best; it is the most serious, and though the author has there one bête noire in the person of Dr. Ward, who suffers vicari

BIOLOGY OF THE FROG. ously for all the sins of “academic psychology," the reader is not wearied, as in the Ethics volume, by The Biology of the Frog. By Samuel J. Holmes, incessant declamation against Nietzscheanism, on the

Ph.D. Pp. vii + 370. (New York : The Macmillan one hand, and what is politely called “hell-fire

Company; London : Macmillan and Co., Ltd., 1006.)

Price 6s. 6d, net. morality” on the other. On psychology our author has nothing very start- IN

N the vast literature that treats of the frog there ling to say He defines his subject as the science, not is no comprehensive summary of its biology. of consciousness, but of mind. He favours the Every natural history teacher has realised this want, Wundtian theory of psychophysical parallelism. He which has increased in proportion with the great regards mind as a product and phenomenon of evolu- recent extension of instruction in elementary natural tion; or rather, having boldly stated that life is prior phenomena. No animal is more thoroughly known to mind, he closes one of two chapters on the evolu- from the anatomical, histological, and embryological tion of mind by maintaining that the responsiveness aspects, but on the side that appeals to teachers and of the leucocyte to irritation points to sentiency on commencing students, the study of habit and function, its part, and by withdrawing his bold statement in existing knowledge of the frog is scattered and often favour of a bolder, that life and mind are co-equal, untrustworthy. This gap the author strives to fil. co-extensive, and of common origin. That is to say, writing primarily for the student. His book is a he levels up the leucocyte to man. In the latter part compilation of what is known of the behaviour of the of his book he dwells much more on the will than frog and of its several organs. Unfortunately it is on the intellectual functions, as he wishes, not to not only this. Dr. Holmes has not freed himself lead up to a text-book on logic, but to the consider- sufficiently from formal and dogmatic zoology. He ation of conduct. The result is that many questions must have all the nomenclature and the anatomy of which one finds discussed in the ordinary handbooks the medical school, as though we could never leam are not even mentioned in this; but, of course, amid or teach zoology without a load of descriptive strutthe multiplicity of cheap introductory works there tural details. The new wine of comparative physiois no reason why all should go in the same ruts. · logy has been poured into the old vessel and has

In the volume on Sociology one notes that our burst it, leaking out now here now there, so that no author follows the Spencerian line that the State has good draught is obtainable. The wine, however, in no consciousness of its own, and therefore the welfare good, and the more pity the framework was not of the State never means anything more or other better adapted to hold it and yield it to the thirsty than the welfare of the citizens. He follows his soul. master, too, very closely in his opposition to free The frog enters on p. 62, chapter ii. Here “we education, which he thinks as bad as free breakfasts begin our study.” L'nfortunately there are for the children. A later chapter is occupied with an earlier chapters, with which most readers will begin indictment of the modern city, and others with a The first deals with the classification of Amphibia, discussion of socialism, conservatism, and liberalism. and ought to have been simplified or postponed. The



second is the main ecological chapter, and involves chapter of the book an idea of the process of evoluthe use of many anatomical expressions that a young tion. student will not understand until later. The mode of Few misprints occur, but “Wiederscheim " for the protrusion of the tongue by lymph pressure; the distinguished anatomist of Freiburg is of irritating changes in the liver, fat-body, and gonads; the form- frequency.

F. W. G. ation of new blood corpuscles and other difficult topics, are referred to before so much as a brief reference to the chief features of the frog itself.

OUR BOOK SHELF. The succeeding chapters are arranged on the stereo- Morphologie und Biologie der Algen. By Dr. Friedtyped anatomical plan. In reference to the external rich Oltmanns. Zweiter Band, Allgemeiner Theil. features, no remark is made of the prevalence or Pp. vi + 443. (Jena : Gustav Fischer, 1905.) Price significance of the dark upper sides and light under

12 marks. sides of animals, or of the meaning of gradational | It is difficult to say of this much-wished-for and longshading: A green pigment is attributed to the frog expected fruit of Dr. Oltmanns's industry more than at the close of the third chapter and denied on p. 192.

that it meets all these wishes and hopes. There is Descriptions of the internal organs, of development,

one respect in which a fault may be found, the last

to be thought of, viz. the arrangement, but it is and of the histology of the different organic systems cured by the provision of an excellent index. Detailed occupy the rest of the book. We have, however, criticism of a work of this size in the pages of NATURE admirable summaries of physiological action under is out of the question, and the present writer coneach histological section, and for these teachers will

fesses that he has attempted such a task several be thankful. The treatment of the skin and of the only left no satisfaction to himself, but kept a fear

times, but always with the result that his effort not blood, of digestion and respiration calls for praise. before him that his judgment might easily be misThe seasonal metabolic changes in the tissues of the understood. frog are well described under the various organs that

In a word, the book is invaluable to all workers are affected, and the references will enable one to find at this subject, and well worthy of the great reputathe original papers with ease.

tion of Dr. Oltmanns as a researcher and teacher.

If any faults were to be found in a detailed criticism The book is one that will prove useful to every they would be, not with Dr. Oltmanns, but with the teacher of elementary biology, and its usefulness fate that has prevented his access to our great collecwould have been enhanced by a thorough-going bio- tions. This short notice of so great a work must logical treatment and simplification of the anatomical not, from its brevity, seem to lack in the heartiness details. Few biological writers realise what

the reviewer wishes to express in his welcome to it. stimulus to teachers and to taught lies in a

The volume has been long needed by those who are

earnestly at work, and no one values it more than mode of presentation of a well-worn subject. In the the writer of this brief note of thanks for it, and for writing of a biology of the frog a superb opportunity the industry of the author of it. has presented itself of boldly embarking on


GEORGE MURRAY. physiological method and of subordinating anatomy Atlas colorié de la Flore alpine. By J. Beauverie and to the working out of function and response. More- L. Faucheron. Pp. 98. (Paris : J. B. Baillière and over, the biology of the frog is not well worn. It is, Son, 1906.) Price 7.50 francs. in contrast with anatomical knowledge, inaccessible | The recollections of botanising expeditions in the and scattered, and with much labour it has been High Alps must ever remain a source of pleasure to brought together for the first time. With so much those who have had such enjoyable experiences. Not novelty at his disposal one cannot help regretting only the botanist, but anyone endowed with a spark that the author has adopted an arrangement for his

of latent appreciation for the beauties of nature can

not fail to be aroused to enthusiasm when for the first work that puts biology into a subordinate place, with

time he has the good fortune to behold patches of the result that he has made a useful but not an Anemone vernalis in the spring, or to discover clumps illuminating work.

of Ranunculus glacialis on the snow-line. It is It is in no carping spirit that we point out a few natural, therefore, that there should be a demand for suggestions and corrections for a second edition.

floras of the Alpine regions adapted to amateurs, and Chiefly we should advise the deletion of the experi

Such is the also worthy of professed botanists.

nature of this volume, which contains excellent illusments and experimental results dealing with severe trations combined with simple descriptions of the lesions. The chapter on the nervous system is one flowers and references to localities where they may that no sensitive student could read without shudder- be found. To confine the book to reasonable coming, and a recapitulation of the revolting experi- pass, only fairly common Alpine plants are included, ments made by certain writers was wholly unneces

and preference is given to the denizens of the higher sary in such a work as this. It is with regret that Alps. So far as the selection is concerned, there is

little to note except that the orchids have received we notice this serious drawback.

rather scant measure, and the thistles are entirely The description of the tadpole, and, indeed, of the omitted. Some of the plants, e.g. Douglasia life-history generally, while fairly careful in cellular vitaliana and Androsace villosa, are interesting for detail, is lacking in any broad suggestiveness that their association with the French Alps, while, on the will remain after the anatomical detail has faded from other hand, several species are included that are the mind. The mode of hatching, the meaning of flects credit on the authors for their clear and pithy

absent from French territory. The compilation refood-yolk, the fish-like character of the larva are not

descriptions, and on the publishers for the manner touched upon, nor is there given in this or any other in which the plates are produced.





they all appear alike as a shade of uniform grey. [The Editor does not hold himself responsible for opinions approaching nearer, differences are observed in the headexpressed by his correspondents. Neither can he undertake

lines; nearer still, varieties in paragraphing come into to return, or to correspond with the writers of, rejected sight, and at a reading distance the figures are all simula manuscripts intended for this or any other part of NATURE,

taneously distinguishable. This experience is partly, but No notice is taken of anonymous communications.]

only partly, applicable to human faces. Those that

alike are certainly distinguishable at shorter distances than Measurement of Resemblance.

unlike ones, and I notice no excessive clustering of values At the distance of a few scores of paces the human face closely round particular values of N in my results, wnich appears to be a uniform reddish blur, with no separate there would be if mistakability always occurred near a features. On a nearer approach specks begin to be seen, particular stage, such as that at which the whites of the corresponding to the eyes and mouth. These gradually eyes cease to be visible, or at twice or three times that increase in distinctness, until at about thirty paces the

distance. features become so clear that a hitherto unknown person A strong likeness in small details may so dominate thcould thereafter be recognised with some assurance. There | perception that a want of likeness in larger features is is no better opportunity of observing the effects of distance

overlooked. Here the distance of maximum mistakability in confounding human faces than by watching soldiers will be small, the portraits appearing more unlike when reat a review. Their dress is alike, their pose is the same,

moved further off, and the small details cease to be visible. the light falls upon them from the same direction, and they

Extreme cases of partial likeness, whether in contour are often immovable for a considerable time. It is then in detail, would, of course, be noted and allowed for. noticeable how some faces appear indistinguishable at

With these exceptions the index of mistakability appears tu distances where great diversity is apparent in others, and be a fair, even, as I think, a close, approximation to an the rudely-defined idea will be justified that the distance at index of resemblance when the quality of the observed which two faces are just mistakable for one another might likeness is recorded by appropriate letters, as will be deserve as a trustworthy basis for the measurement of re

scribed later on. semblance. The same may be said of obscurity, of con- The observational value of mistakability lies in its usko fused refractions, and of turbid media ; but in this letter ing a simple question which different persons would answer I shall confine myself almost wholly to the effects of in the same way, when they had become familiar with distance under the conditions of ample light and a trans- the method. On the other hand, likeness includes mutual parent atmosphere. Beyond this I shall say nothing, except suggestibility, a highly complex perception dependent on in one paragraph almost at the end.

the mind of the observer, and consequently appreciated The scale of the features has, of course, to be taken differently by different observers, as is notoriously the case. into account. This is of much less importance in living The apparatus I now use with ordinary photographs acts persons than in portraits, because the differences in scale very well, but I wasted much time before I contrived if, of the adult human face are not very great, whereas those and more before sending it to be made in a workmanlike in photographs and paintings-ranging as they do between

I think it could still be improved, so I will deminiatures and life-sized portraits-are so. It is necessary scribe, not my own, which was made for me by Baker, to adopt a facial unit, based on some specified dimension. 240 High Holborn, but such as I should order if I required That which I use is the vertical distance between the another one. middle of the line that joins the pupils and the parting It is a long, thin, light box or framework 61 cm of the lips. It is unaffected by head-dress or by the thick- (2 metres) long, 10 inches (25 centimetres) wide, and ness of the hair on the top of the head, while its inches (5 centimetres) deep, which admits of being lower termination can be located in bearded face divided for sake of portability. It stands on two folding more accurately than the chin. I call this u.

supports 2} feet apart, which fold back when out of use: traits have different units, they are distinguishable as when in use they can be clamped to any ordinary table. u and u'. If d and d' be the critical distances at which These raise the long box in a sloping position, the end mistakability first occurs, then ud and u' d' are neces- towards the eye being at the most convenient height for a sarily equal, and either of them would serve as a measure person seated on a chair, but the further end being lower, of mistakability; but as u is very much smaller than d, because it is easiest to look somewhat downwards.

150 this fraction would always be a decimal preceded by one rollers, A and B (Figs. I and 2), run independently on a

Therefore I take the index of mistakability, horizontal axis at one end of the box, and two corresprindwhich I will call N, as = 1000 uld. It is, however, con- ing ones, a and b (Fig. 2) at the other end. A light slegs venient to measure u and d by different scales; u in milli- that slides on the top of the box is harnessed in front to a metres, distinguishing it as Umi din centimetres, dis- tape graduated in centimetres, which passes over and round tinguishing it as de Then N= 100 unlite.

A, back to and around a, and thence forwards to the back Of course, N could be expressed by the arc or angle of of the sledge. (By inadvertence the path of the tape which uld is the chord, but it would be a roundabout between the lower margins of A and a has been omitted method, as angles could not be measured directly without in Fig. 1. The reader might dot it in pencil.) A similar special and troublesome apparatus.

I find it very con- sledge and tape is adapted to B and b. The tapes lie half venient for my purposes to employ a nomenclature for an inch above the box (Fig. 1), and can be manipulated by chords based on that of the metrical system, d, the dis- the hands severally, so either or both sledges can be easils tance, being the radius or rad.” So a chord=1/100 be- pulled either backwards or forwards while sitting in the comes a “centirad, and that = 1/1000 a “millerad.” A chair, and their distances from the rollers at any moment he centirad is the chord of 34:4 minutes of a degree, and, there- read off on the graduated tapes. (A winch and handle are fore, a trifle larger than the apparent diameter of the sun superfluous.) The photos are mounted on two easily de

It is equal to the apparent size of one-tenth of tachable standards (Figs. 1, 2), with clips at the bottom to an inch at 10 inches distance from the eye, which is a hold them (not shown in the diagram), and standing on convenient distance for reading small type. A millerad circular bases. These fit quite loosely into shallow bcllas which subtends between three and four minutes of a degree, in the tops of the sledges. The standards can be lived and is equivalent to 1/100 of an inch seen at 10 inches, is as out, the photographs inserted, and the whole replaced with small an interval as can usually be detected in photographs perfect ease. The circularity of the bases of the standaris without scrutiny, though a normal eve is able to distinguish enables either of them to be set a little askew, which is coone-third or one-fourth of that interval between venient when the broad, full face of one portrait has to be sharply defined objects.

compared with the narrowed, three-quarter face of anotha. Mistakability is only an approximate measure of resem- A board stands vertically across A and B, and above them as blance, for it depends more on the scale of the distinguish- a bridge. An eye-slit of half an inch width runs below its ing features than on the amount of difference of those upper edge (Figs. 1, 3, 4), through which the photos are features. This peculiarity is well exemplified, though, viewed, and from which the distances of the sledges are greatly exaggerated, by what is seen in the time-tables reckoned. A ledge 1 inch below the eve-slit (Fig. 1), with hung up by railway stations. From across the road, say, a parapet a little less than 1 inch high. fcrits a lins,



If the por

or two zeros.

Or moon.


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