May 30, 1878] 



NATURE 



115 



purposes of an oviduct, and of a portion of the middle 

 tubules to those of sperm-ducts is what the observations 

 of Balfour and Semper have established — and more 

 especially the open funnel-like character of the tubules to 

 begin with. 



Minor details and important confirmations of the more 

 familiar facts of Vertebrate development I have not space 

 to mention here, the whole series of embryonic pheno- 

 mena is described with more or less detail by Mr. 

 Balfour, and I have singled out the more striking facts 

 and speculations of the monograph for brief notice. 



In commenting on such a work as this, it is strongly 

 brought to one' s perception that the method of publication 

 of the results of such laborious investigations is necessarily 

 very imperfect— and subject to a serious deficiency in 

 logical continuity and artistic effect. Mr. Balfour has 

 studied the very widely-diverse phenomena of interest 

 which the developing Elasmobranch presents from the 

 first separation of the egg up to the nearly complete 

 formation of all its organs. In order to state all the 

 different results he has obtained, he is obliged, as is 

 usual in embryological monographs, to give them in his- 

 torical sequence. To the experienced student of embry- 

 ology this method of statement and the presentation of 

 drawings copied from actual sections and specimens is 

 sufficient. It would be impossible to publish observa- 

 tions within a reasonable period of the date of investiga- 

 tion by pursuing any other method of statement. And 

 yet the monographical and historical method, together 

 with the objective "nature-true" drawings of sections 

 is one which prevents an author from fully exhibiting the 

 import of his observations, and from duly imparting to 

 the reader in a clear and simple form what is, after all, 

 the thing which the reader desires to know, namely, what 

 is the net result of such observations in relation to the 

 great questions of morphology. The fact is, there is no 

 such thing as a science of embryology ; it is not even a defi- 

 nite branch of a science. The development of organic form 

 is a necessary part of the science of Organic Morphology, 

 and the results of the study of development can be given 

 with full clearness and in an intelligible manner only 

 when formulated as parts of the general doctrine of the 

 science under which they fall. The conclusion from this 

 is, that the great value of Mr. Balfour's work will not be 

 fully appreciated or rendered clear to the majority of 

 zoological students until they are re-stated, not from the 

 monographical standpoint ; but from the more general 

 point of view of Animal Morphology. This more syste- 

 matic exposition of his Elasmobranch studies and of other 

 like researches in combination with a general survey of 

 the morphology of all groups of the Animal Kingdom as 

 revealed by their developmental histories, we may expect 

 before long to receive from Mr. Balfour himself in the 

 form of a continuation of his well-known Elements of 

 Embryology. E. Ray Lankester 



OUR BOOK SHELF 



Gold. By Edwin W. Streeter, F.R.G.S. Fifth Thousand 

 (London : Chapman and Hall.) 



The lettering on the cover of the book, as giren above 

 •will hardly prepare the reader for the statement on 

 the title-page, that the work is a translation and abridg- 

 ment of Herr von Studnitz' "Die gesetzliche Regelung 



des Feingehaltes von Gold und Silber-Waaren," by Mrs. 

 Brewer, with notes and additions by Mr. Streeter. 



The work itself contains information which it is useful 

 to possess. It embodies brief abstracts of the law 

 of various countries concerning the standard of gold 

 and silver wares, and discusses the question whether 

 the manufacture of articles in the precious metals should 

 be subject to legal control. Mr. Streeter' s notes occupy 

 10 out of the 150 pages. He states that the system of 

 "Hall-marking" was "instituted on the supposition 

 that the assay and test of precious metals was a matter 

 too recondite to render a power of adequate discrimina- 

 tion for so valuable a transfer of property a thing reason- 

 ably to be expected of the public generally." 



This is a very obscure way of saying that, as the value 

 of gold and silver wares could not be recognised by in- 

 spection, it was advisable that all articles should be 

 stamped by authority. The necessity for such control has 

 long been felt, and it was well justified in 1677 by the 

 author of the "New Touchstone for Gold and Silver 

 Wares," who says : "The truth is, the gain by adulterat- 

 ing gold and silver works is so sweet and enticing that 

 what excuse will not these adulterators find that they may 

 have their unlawful liberty.' ' 



In London the control has been wisely vested in the 

 Goldsmiths' Company since the fourteenth century, and 

 in the country there are several assay offices which were 

 reported on by a Select Committee of the House of 

 Commons in 1856. Mr. Streeter urges that gold of one 

 standard only — 18 carat — should be used, or that if other 

 alloys are employed the tradesman should " mark them 

 with his own name, state the value of the composite 

 matter, and trust to his genius for the sale." Trusting 

 to genius for the sale of articles is all very well, but the 

 practice of a person stamping the wares he sells with his 

 own mark surely affords no protection against the fraudu- 

 lent tradesman as the marks are not likely to outlive the 

 age in which they are impressed, and would be as readily 

 counterfeited as those of a responsible authority. It 

 should also be added that the initial or distinctive mark 

 of the maker of an article of gold or silver is already 

 included in the Hall mark. 



LETTERS TO THE EDITOR 



\The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to returny 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters at 

 short as possible. The pressure on his space is S0 great that it 

 is impossible otherwise to ensure the appearance even ef com- 

 munications containing interesting arui novel facts.^ 



Alternate and Stereoscopic Vision 



With reference to Mr. Galton's observation in his instructive 

 paper in Nature, vol. xviii. p. 98, that "sometimes the image 

 seen by the left eye prevails over that seen by the right, and 

 vice versd," I may mention that, as I had noticed some years 

 ago, this may be best observed without a stereoscope. If on 

 looking at any object a few feet distant, a nearer object be 

 placed about midway between the first and the eyes, there will of 

 course be two images seen of the near, when the eyes converge 

 on the distant ; one of these images seen, say by the right eye, 

 overlaps the distant object as seen by the left eye, and if the two 

 objects be about equally illuminated (or the near one rather the 

 brighter), the overlapping image will alternately solidify and 

 disappear, according to the alternate waxing and waning of 

 sensibility in the eyes. This alternation may be made at will, 

 by desiring to see the near or the distant object ; the fluctuations 

 take place about every ten seconds normally, but the changes 

 may be willed (though not so completely) as often as every 

 second. 



If the observer can see stereoscopically without an instrument, 

 i.e., can dissociate the usually coincident motions of focussing 

 and convergence, this alternate action of the eyes is seen very 



