November 9. 1899] 



NATURE 



29 



part of the night between the 14th and I5lh, as well as through- 

 out the night Ijetween the 15th and i6th of this month, begin- 

 ning the watch at 10.30 p.m. Before that hour observations 

 : cannot be made, the radiant point of the Leonids being below 

 our horizon. 



A computation of the perturbations of two other stations in 

 the stream, in which we have made use of Dr. Berberich's 

 results to supplement our own, has established a fact which we 

 had anticipated, viz. that different parts of the immensely long 

 ortho-stream have been so variously affected by perturbations 

 that the stream cannot now be a uniform one evenly extended 

 along a portion of its elliptic orbit. We must accordingly 

 recognise that it is more or less sinuous, and that, moreover, the 

 distribution of meteors along it is uneven. All such circum- 

 stances introduce further elements of uncertainty into any 

 attempt which we may make to form a forecast. 



On account of the abnormal amount of the perturbations 

 within the last thirty-three years, the method 6y which the 

 prediction was made in 1866 is not sufficient on the present 

 occasion. It was based on the average amount of the shift of 

 the node. If employed on this occasion, it would assign for 

 the shower of this year the epoch 1899 November I4d. I2h., 

 which is almost certainly too early. 



London, November 4. G. Johnstone Stoney. 



A. M. W. Downing. 



Undercurrents. 



I AM much obliged to Admiral Makaroft for so courteously 

 answering my queries in my letter to Nature in the number 

 for August 3, p. 316 ; and I regret that having mislaid my copy 

 of his book, the " Viliaz," I have not been before able to reply. 

 On now studying his observations in the Strait of Bab-el- Mandeb, 

 and his remarks in his letter (Nature, October 5, p. 544), I 

 fear I cannot any more share his opinions than I did before. 



My point is that it is not sufficient to ascertain that there is a 

 difference of specific gravity either between the surface water 

 or either side of a strait, or between the surface and lower 

 strata of water in a strait, to be able to come to the conclusion 

 that such difference is the primary cause of surface and under- 

 currents in opposite directions. It has been shown by experi- 

 ment that such differences give rise to a slow interchange of 

 water, and to this extent I am of course prepared to agree that 

 differences of specific gravity cause opposing currents; but the 

 currents we are dealing with are of a vastly different character 

 and strength. 



I have already pointed out that my observations in the 

 Dardanelles and Bosporus in 1872 showed that the currents 

 did not always run in the normal directions, and that their 

 variations were traceable to the varying winds ; and to that I have 

 nothing to add. 



On looking at Admiral Makaroff's density observations in the 

 Strait of Bab-el-Mandeb, I see that the specific gravity varied 

 from I 0279 at the surface to i 0292 at 200 metres, while the 

 surface waters of the Red Sea itself and of the Arabian Sea near 

 Sokotra are given as I '0300 and i "0279 respectively. 



In saying that " here are none of the differences of specific 

 gravity demanded by Admiral Makarofi^s hypothesis," I was 

 referring to the great contrast between the difference in the 

 densities of the Black Sea and Mediterranean, viz. 017, and 

 those of the Arabian Sea and Red Sea, viz. '002, and never 

 thought for a moment that the very small variation in density 

 in the latter case could be held capable of setting up currents 

 of i^ knots in opposite directions at surface and bottom, as 

 found by " Stork's" observations in the N.E. monsoon. 



No observations have yet been made on the undercurrent in 

 Bab-el-Mandeb in the S.W. monsoon ; but the surface current 

 is known to run in the contrary direction to what it does in the 

 N.E. monsoon, i.e. again with the wind, or out oi the Red 

 Sea, and I should be much surprised to find that the under- 

 current does not also run in the contrary direction, probably 

 with much greater strength than the surface current, because 

 the great evaporation of the sea has also to be made up. 



Absolute proof of the causes of such phenomena as these 

 under discussion comes slowly, and only after laborious observ- 

 ation ; but I certainly think that the work of the last twenty-five 

 years has tended to show that the influence of density as com- 

 pared with wind is insignificant. W. J. L. Wharton. 

 Florys, Wimbledon Park, November 4. 



WO. 1567, VOL. 61] 



" Anlage " and " Rudiment." 



Some months ago Prof. Herrick, who is in charge of the de- 

 partment of Neurology in the "Dictionary of Philosophy and 

 Psychology," which I am editing (now in the press of Macmillan 

 and Co.), addressed a circular to various authorities asking their 

 opinion on certain matters of terminology. The results were 

 collated and discussed by Prof. Herrick in {he/oitrn. of Comt. 

 Neurology, vii. 3-4, 1898. Among the matters in question 

 was the English equivalent of the German term Anlage. 

 Prof. Herrick came to the conclusion that Proton^wd. Rudiment 

 were more available than any other words suggested (ruling out 

 the use in English ol Anlage with its German inflexion). 



It now happens that the French and Italian committees, who 

 are recommending equivalents, in their respective languages, for 

 the terms in the Dictionary, make reports which I think are of 

 importance. Prof. Delage, of Paris, for the French committee, 

 recommends ruditnent, and, as it happens, Prof. Morselli, of 

 Genoa, sends in rudimento as the preferred Italian term. This 

 agreement — and to say this is my aim in writing — affords a strong 

 argument for the adoption of Rudiment in English. It is 

 evident that it would be of immediate and very great advantage 

 — for example, to translators from any one of these languages 

 into any other— if Rudiment were made the common rendering 

 ol Anlage in the three other languages of modern science. The 

 other great advantage would be that we already have the adjective 

 form, rudimentary, in use. 



Furthermore, the psychologists may use the same term for 

 \\\& Q&xmSiW psychologische Anlage ^\\\c^ has crept into recent 

 German discussions. In English, biologists and psychologists 

 will then have the common term rudiment with a well-under- 

 stood signification. I am recommending this to the committee 

 on terminology of the American Psychological Association, of 

 which I happen to be secretary. J. Mark Baldwin. 



Oxford, October 22. 



Interference Curves depending on Perspective. 



Closely allied to the halo round an observer's shadow 

 (referred to in Mr. S. Newcomb's letter in Nature, October 5) 

 are a number of phenomena due to perspective, which may be 

 seen every day by any one who is on the look out for them. 



Among these may be mentioned the dark waves which seem 

 to accompany a traveller when he looks through two series of 

 upright palings which lie parallel to each other and his course ; 

 also the patterns like the grain of wood which appear when two 

 superposed sheets of gauze are held against the light. 



As these and the like appearances have not, as far as I know, 

 hitherto been looked at from a mathematical point of view 

 except in one instance,^ the three following examples, which 

 are typical but simple, may be of interest : — 



(i) Interference rings due to parallel lines on a spherical 

 surface, and their shadow or reflection on a plane or in a plane 

 mirror. 



Let a small part of a sphere of radius r be ruled with equi- 

 distant parallel lines, the distance between the lines being 

 small compared with r. Let the convex surface of the sphere 

 touch a plane mirror, and let surface be viewed from a distant 

 point, the line joining the distant point, and the point of 

 contact making an angle i, with the normal to the plane. 



Taking the plane containing the point of view and the normal 

 through the point of contact as the plane of reference, let a be 

 the angle between the plane containing a line and its reflection 

 and the plane of reference. Let 9 be the angle which a point 

 on the spherical surface distant p from the normal through the 

 point of contact subtends at the centre of the sphere (so that 

 r sin = p), and <p the angle which p makes with the plane of 

 reference. 



It is plain that where, from the point of observation, any part 

 of any line hides the reflection either of itself or any other line, 

 the field will look brighter in that direction than where the line 

 and the reflection are both visible, and the condition which 

 must be fulfilled in order that one line may hide the reflection of 

 another n lines off is (neglecting second order quantities) that 

 the distance between the hidden reflection and the line reflected 

 (that is, twice the distance of the line from the reflector) multi- 

 plied by tan i should be equal to n times the projection of a on 

 the plane of reference, or in symbols, 



2r(i -cos tf) tan i= , 



^ cos a 



1 Lord Rayleigh's " Theory and Manufacture of Diffraction Gratings " 

 (.Phil. Mag., 1876). 



