544 



NATURE 



[April 6, 1893 



Institute, and is printed in the Section's Proceedings for 

 M arch. 



A SIMPLE contrivance for determining the refractive index 

 of a liquid without the use of a circular scale or a hollow glass 

 prism, is described in Wiedemann! s Annalen by Mr. H. Ruoss, 

 of the Stuttgart Technical High School. The liquid is poured 

 into a rectangular vessel, closed on one side by a plane-parallel 

 glass plate. A small plane mirror is half immersed in the 

 liquid, and mounted so that it can be placed exactly parallel to 

 the plane-parallel side, A telescope is directed towards the 

 mirror from outside, about 4 m. distant, its axis being normal to 

 the glass side. To this telescope is attached at right angles a 

 scale 3 m. long. On looking through the telescope the image 

 of the scale in the mirror appears broken into two by the sur- 

 face of the liquid, the lower image being formed by rays which 

 have undergone refraction and reflection in the liquid. The 

 divisions on the cross-wire measure the tangents of the angles of 

 incidence and refraction respectively, which, since both the sets 

 of rays after reflection are parallel, determine the refractive 

 index of the liquid. A correction has to be. applied for the 

 thickness of the plate-glass, and it is best to make the angle of 

 incidence as large as possible. Before taking the readings, the 

 instrument should be adjusted by making the cross-wire coincide 

 with its two reflections in the mirror and the plate, and placing 

 the scale in a parallel and horizontal position with its reflected 

 zero on the cross wire. With these adjustments and corrections 

 the apparatus is capable of giving very accurate results. The 

 angles can be measured to within 5", and a large number of 

 readings may be taken with different inclinations of the mirror. 

 A set of five measurements for water in sodium light, for 

 instance, gave a refractive index of i "33276, which coincides 

 with Walther's value to the fourth decimal place, and is subject 

 to a probable error of 0*00003. 



At the magnetic observatory of Potsdam some interesting 

 improvements have been made in registration of the needle's 

 variations, a brief account of which is given by Herr Eschen- 

 hagen {Met. Zeits.). He uses a greater length of abscissae than 

 usual (20 mm. per hour), and obtains a fine curve by cutting off 

 the border rays by means of a paper screen on the lens, by de- 

 termining exactly the chemical focus, and by use of a very 

 small mirror. The slit is 0*25 mm. In the case of great mag- 

 netic disturbances, trouble sometimes arises from the movable 

 light point going beyond the recording surface, even where, as 

 in Potsdam, this has a width of 190 mm. (7 "6 inches), so that the 

 most interesting parts of disturbances may be lost. An attempt 

 was made to remedy this with prisms of a certain angle of re- 

 fraction, but there are objections to this plan. A more simple 

 and effective method was hit upon ; the magnetic mirror is made 

 in three parts, or facets, inclined to each other at an angle of 

 3". It is enclosed in a bell-jar, in which the air is kept dry and 

 tree from sulphur vapour. The mirror gives three beams, of 

 which usually only the middle one is concentrated in a fine light 

 point on the drum. During a strong disturbance, and just 

 before this light point leaves the drum, another point appears 

 on the opposite side, which takes up and continues the record. 

 These and other improvements will be described in detail ere 

 long in publications of the Observatory. 



According to recent researches by M. T. J. van Beneden 

 on the fossil Cetacea found in the regions of the Black Sea, the 

 Caspian, and the Sea of Aral, the basin of the Black Sea con- 

 tains all those forms which to-day characterise ocean fauna 

 ( Balsenides, Ziphioides, Delphinides, and Sirenides) ; and 

 taking also the region of rivers now flowing into that sea into 

 account, it is probable that the whole of Central Europe at the 

 end of the Miocene period was traversed by numerous arms of 

 the sea, the Black Sea reaching to Vienna, Linz, and even to 

 NO. 1223, VOL. 47] 



the Lake of Constance. Towards the end of the Pliocene, or 

 the beginning of the quaternary period, owing to considerable 

 depressions, the Straits of the Bosphorus were formed, and the 

 water of the Mediterranean pressed into a basin formerly con- 

 nected with the Arctic Sea. Thus the passage of a new fauna 

 was made possible, which gradually, under favouring conditions, 

 displaced the older. The Caspian was separated before the 

 new forms had spread so far, and we find in it fifty-four species 

 of fishes, which are neither in the Sea of Aral nor the Black 

 Sea, and only six species which it has in common with those 

 two others. 



From recent researches on transference of material in plants 

 (represented, e.g. by transference of starch in the potato), Herr 

 Brasse is led to present the following view of what goes on : 

 The assimilation of carbon in the sun's rays is manifested 

 directly in deposition of starch in the chlorophyll grains. 

 Through action of diastase in the leaves, and at a temperature 

 lower than that of its formation, this starch is changed into 

 reducing sugar, which spreads by diffusion from its place of 

 formation into all the tissues of the plant. In certain parts, 

 and especially in the tubers, the sugar is continuously trans- 

 formed. The tubers, with regard to dissociation, act like the 

 cold wall in vaporisation of a volatile liquid in an enclosed 

 space. The sugar-content of all cells of the plant seeks to enter 

 into equilibrium with that of the cells of the tubers, in which 

 the content is less, because a change of sugar into starch takes 

 place, and the coefficient of this change is here less than that 

 of the converse change in the leaf, the temperature of the tuber 

 being less. Owing to this inequality, there is a transference 

 of starch from the leaf into the tuber, in which it passes through 

 the intermediate stage of sugar. In a similar way Herr Brasse 

 would explain the transference of nitrogenous and mineral 

 plant materials, and their storage in special organs {Comptes 

 Rendus de la Socicte de Biologie). 



Mr. E. Lommel has succeeded in fixing photographically the 

 equipotential lines due to a current flowing through a conduct- 

 ing sheet. A current of 20 amperes was sent through sheets 

 of copper 0"5 mm. thick and of various forms. The sheets 

 were covered with sensitive paper strewn with iron filings, 

 which arranged themselves along the lines of magnetic force 

 due to the current, or, what amounts to the same thing, the 

 lines of equal electrical potential along the conductors. The 

 configurations thus obtained were fixed by holding a lighted^ 

 match for a few seconds above the paper, yielding on develop- 

 ment a beautiful representation of the flow through the current 

 sheets. Two of these figures are reproduced in the last number 

 of Wiedemann s Annalen. One of them represents the flow 

 through a ring formed by two concentric circles, the current 

 being conveyed by wires soldered to two diametrically opposite 

 points. The other exhibits the equipotential lines in a rectangle 

 with a hole in the middle and wires soldered to two opposite 

 corners. A consideration of the various ways in which the 

 presence of a strong magnetic field affects the configuration of 

 the lines observed has led the author to a possible explanation of 

 the " Hall effect." This phenomenon is only produced by mag- 

 netic lines of force running in a direction normal to the plate, or 

 by the normal component of slanting lines. If in a rectangular 

 current sheet made of diamagnetic material two points at 

 equal potential, but on opposite edges of the sheet, be con- 

 nected with a galvanometer, no current will be indicated until 

 the sheet is brought into a strong magnetic field. According to 

 Weber's theory of diamagnetism, currents are then generated 

 in the molecules opposite in direction to the amperian currents. 

 These molecular currents give rise to a resultant current round 

 the edge of the sheet, strengthening the ordinary current on 

 one side and weakening it on the other. This state of things. 



