38o 



NA TURE 



[August 17, 1893 



of the exposure, but if photographs obtained with longer expos- 

 ure, are utilised for the determination of the relative position of 

 stars, it will be necessary to know what star on the plate was 

 used as guider, and the distortion by refraction must be investi- 

 gated for all stars at any considerable distance from it." From 

 this it will be seen that the photographs of stars obtained for the 

 determination of parallax, or in connection with the star cata- 

 logue, are unaffected by the result, since the exposure in each 

 case is usually less than the limit defined by Prof. Rambaut. 



Astronomy Popularised. — We have previously referred 

 to a proposal to issue a new astronomical periodical, designed 

 for amateurs, teachers, students of astronomy, and the public 

 generally. The first number of this Popular Astronomy will 

 be published about September I, by Mr. W. W. Payne, 

 Goodsell Observatory, Carleton College, Northfield, Minn., 

 U.S. Messrs. William Wesley and Son, 28 Essex Street, 

 Strand, London, are the agents for England. The periodical 

 will be issued monthly, hut no numbers will be published for 

 July and August of each year. One of the features will be 

 a scheme of work suitable for a small telescope, field glass, 

 opera glass, and the naked eye. Those who wish to know 

 their way about the .sky will find their wants supplied, and 

 home readers will be catered for by means of lists of best books 

 and schemes of study. From these and other matters men- 

 tioned in the prospectus it seems probable that tlie periodical 

 will possess the features that command success. 



Comet Appearances in the Year 1892. — Prof H. Kreuiz 

 has collected together all the appearances of comets during the 

 past year, this list appearing in the Viet teljahrschrift der Astro- 

 nomischen GeseUschaft, 28jahrgang, parts I and 2. In addition 

 10 short descriptions of ihe appearances put on by them at the 

 limes of discovery, and to the values of the elements of 

 the new ones, he gives references to all the observations that 

 have been made of ihem. Among those that receive more than 

 usual attention are Comet 1892 I., discovered by Swift ; Comet 

 Holmes (1892 III.), VVinneike's Comet (1892 IV.), and Comet 

 1892 V. (Barnard), since it was the first (ixcluding that photo- 

 graphed in the Solar Eclipse of May 17, 1882) discovered by 

 photography. 



GEOGRAPHICAL NOTES. 



The Society for the study of French Congo has organised a 

 strongly-manned expedition to survey the. valley of the Kuilu 

 and Niadi rivers, in order to ascertain the feasibility ol con- 

 structing a railway from the coast town of Loango to Brazzaville 

 on Stanley Pool. A geological and botanical staff accompany 

 the survey party, and the whole is under the command of M. A. 

 Le Chatelier, who, with fifteen French members of the expedi- 

 tion, sailed from Marseilles last week. 



Russian authorities are determined this year to te.-t the capa- 

 bilities of the Kara Sea route to Northern Silxiir. A small fleet 

 of three vessels, specially built on the Clyde for navigation on the 

 Upper Yenesei, has recently set out in charge of Russian naval 

 (ifficers, who are confident of making a rapid journey. Capt. 

 Wiggins is also in charge of some vessels laden with railway 

 ma crial for the great trans-Siberian line, wr.ich are now on their 

 way 10 the Kara Sea. Dr. Nansen, in the Frani, must now be 

 very rear the entrance to the Kara Sea, and the nature of the 

 ice there will determine which of the three routes into the sea 

 will be attempted. The ultimate establishment of a commercial 

 steamer service is only a matter of money. 



In a racy little pamphlet, La Geographies dans Its Chaires de 

 lUniversile, Dr. Maurice Viguier makes a raid on a number of 

 elementary text-books published by the leading geographical 

 professors of Paris, and he succeeds in showing many errors of 

 statement which should be set right. He goes on to argue that 

 the inaccuracy of these popular schoolbooks, written to satisfy 

 an arbitrary syllabus, proves the geographical incompetence of 

 the authors. Few eminent men in any country could stand such 

 a test, and in truth the faults cited and held up to ridicule so 

 cleverly are faults of composition rather than of fact, and the 

 wors". are due to the ambiguity of words in common speech. 



Prelimi.nary arrangements are being made for the meeting 

 of the Sixth International Geographical Congress in London in 

 1895. This congress will be under the pa'ronage of the f^ueen, 

 and «ill bring together the geographers from all countiies for 



NO. 1942. VOL. 48 j 



the discussion of questions in which the international or univer- 

 sal side of geography will be kept to the front. The month of 

 the proposed meeting has not yet been fixed. 



The Central African telegraph line, projected by Mr. Cecil 

 Rhodes, has been already commenced, and contracts have been 

 signed for its construction from Fort Salisbury as far as Lake 

 Nyasa. The wire will be carried on iron poles, and taken across 

 the Zambesi (a distance of about half a mile) overhead at a 

 height sufficient to allow the traffic on the river to pass entirely 

 unimpeded. The advantage of this line in bringing the 

 region of the great lakes into telegraphic touch with Europe 

 will be very great. 



CHARPENTIER'S EXPERIMENTS DEMON- 

 STRATIVE OF AN OSCILLATORY PRO- 

 CESS IN I HE ORGAN OF VISION AND 

 OF ITS DIMENSIONS. 



^^NE of the fundamental positions in Hering's physiological 

 ^^ theory of visual sensation is that each sensificatory datum 

 diffuses in the retino-cerebral organ beyond its precise locus of 

 incidence, and thus directlymodifies contiguous sensificatory dala. 

 A very elegant experimental substantiation of this position is 

 contained in two simple optical observations by Charpentier' (of 

 Nancy) giving not only the clearest possible demonstration 

 of the fact itself, but an approximate measure of the physiological 

 duration and velocity of the phenomenon. 



These experiments are (l) that of the "black sector," demon- 

 strative of a retino-cerebral oscillation ; (2) that of the " fluted 

 band," demonstrative of the propagation of that oscillation. 



( i) Charpentier s expa'iment oj ihe Black Sector. — A black disc 

 with a white quadrant, revolving once in tw o seconds, illumin- 

 ated by a very bright light (preferably direct sunlight). 

 Observer's eye fixed upon centre of disc. A narrow black sector 

 appears on the white quadrant near the receding edge of the 

 black surface. This is interprelable as a rebound effect in- 

 dicative of an oscillatory process in the retino-cerebral organ ; 

 the first effect at the arrival of the white border is the sensation 

 of white, and this first effect is followed by an after-effect that is 

 black. On closer examination it may be noticed '.hat the angular 

 breadth of the black sector is equal to the breadth of the «hite 



'/^0=™S£C 



interval between it and the receding black border, and that 

 these breadths increase anddiminish with increase and diminution 

 in the speed of revolution ; estimating from this speed and from 

 the apparent extent and position of the black band, Charpentier 

 finds that the white phase and the black phase have each a 

 duration of '014 to '016 second, i.e. that a total oscillation of 

 the two phases lasts '028 to '032 second, ;' ^. that the oscillation 

 frequency is 36 to 31 per second. 



Nothing can be clearer and more striking than this experi- 

 ment ; provided a s'.rong light is used, it can be roughly 

 demonstrated without any elaborate apparatus ; it can be done 

 by a black and white disc slowly turned round by hand, or by a 



J Coinptes Reniins, Soc. Biol.. Mai 16. =3, '30, 1891. CompUt 

 Reniiiis, Acad. Sc, Juillet 27, 1891. Arcli. </e Physiol., Juillet e< 

 Octobre, 1892. 



