covered with a cyanin film, and spread to a spectrum of the length of 23 cm., extending from λ= 650 μp to λ=240 μμ*. An examination of my plates taken quite recently gives the same results.

I

To my thinking the explanation of this seeming discrepancy is very simple: Wood has used cyanin of a different chemical constitution. Kundt and Wernicke found the absorption-bands of Fuchsin to vary, and in a former paper I already have attributed this fact to the same cause. therefore have always given the exact chemical constitution of the preparations used by me. This opinion is corroborated by the fact that the dispersion-curve of Wood's cyanin is quite different from mine, beyond the faults of observation.

Wood greatly undervalues the perfection of my prisms (not Wernicke's as Wood says). It is not at all as impossible as he thinks to make prisms with perfect optical surfaces, or, at least, perfect enough for the purpose. This can be easily seen in my different determinations of the angle of the prisms; their greatest error, for the three last prisms, is +0.25 second. Besides, the principal fault of the method is not this, but the broadening of the image of the slit in the region of the absorption-band, caused by the decline of the dispersioncurve, and the narrowness of the transparent part of the prisms. This fault is the same in Wood's measurements as in mine. That his measurements really are not more accurate than mine, is to be seen by a comparison of the errors of the observations. Nevertheless Wood's thick prisms, which are not transparent for the light in the region of the absorptionband, seem to be very suitable for demonstrations.

My endeavour to apply the refraction- and extinctioncoefficients, found after my method, to a proof of the KettelerHelmholtz dispersion-formula, has given very satisfactory results. Greater accuracy, especially the exact determination of the constants of the formula, will only be possible, as I have shown (l. c.) if we can determine the third decimal of the above coefficients. This is impossible with Wood's prisms as well as with my own. Furthermore, there is no doubt, as I have shown (l. c.) that the indirect methods, based on Cauchy's formulas for metallic reflexion, are simpler and more rational, and equally accurate for further investigations of this subject.

*Wied. Ann. lxv. p. 198.
Ibid. lxv. p. 228.

† Ibid. lvi. p. 424.

XXX. The Magnetic Effect of Electric Convection.
To the Editors of the Philosophical Magazine.

GENTLEMEN,

RE

Trinity College, Cambridge.
Aug. 5, 1901.

EFERRING to Dr. Crémieu's reply in your August number to my article in your July number, I have to apologize for having (owing to an oversight in reading his short note in the Comptes Rendus) represented Dr. Crémieu as using a vertical bar-magnet merely, whereas he actually used a rectangular iron circuit with magnetizing coils on one of its sides.

I cannot agree, however, with Dr. Crémieu in thinking that my objection to his first experiment falls to the ground in consequence of this oversight on my part.

It is easy to show, using Dr. Crémieu's data, that nearly all the magnetic lines of force in his apparatus must have passed through the air and not round the iron rectangle as he supposes. Consequently my objection to his experiment still holds good; and in fact this experiment, if it proves anything, proves that the electromotive force due to varying magnetic induction does act on a static charge. I think that if Dr. Crémieu were to repeat this experiment, using merely a bar-magnet instead of his rectangle, he would still obtain no effect; whereas if he is right and the effect in question does not exist, he ought to obtain a deflexion due to the action of the magnetic field on the current when the disk is charged.

With regard to the rest of Dr. Crémieu's reply, I do not think that what he says in any way affects my objections. My suggestion that the insulation of his sectors was not good enough, is only a very small part of my criticism of his experiments with rotating disks.

Since I wrote the article in the July number, Dr. Crémieu has published an account of an experiment which he supposes proves very conclusively that " open currents can exist.

In this experiment a disk of ebonite 2.5 millimetres thick carrying sectors on one side was rotated rapidly. Near the other side of the disk a charged metal sector was placed so as to induce charges on the sectors as they passed by it. The motion of these induced charges constituted the "open" part of the current. The charged sector was 2.5 millimetres distant from the disk, and its potential was about 115 electrostatic u nits.

This potential is sufficient to produce a spark 1 centimetre. long, so that of course a brush-discharge would occur to the

ebonite, so that it would get a charge on the side next the fixed sector equal and opposite to that induced on the moving sectors. Consequently the total charge on the disk would be zero, and so no magnetic field would be produced.

I am, Gentlemen,

Your obedient Servant,

HAROLD A. WILSON.

XXXI. Notices respecting New Books.

Rapports Présentés au Congrès International de Physique, 1900. Rassemblés et publiés par Ch.-Ed. GUILLAUME et L. POINCARE, Secrétaires généraux du Congrès. Paris: Gauthier-Villars, 1901. Tome I. pp. xvi+698; Tome II. pp. 570; Tome III. pp. 620. THE dawn of a new century forms a fitting occasion for retrospects.

Of such retrospects we have had abundance, but surely few subjects can be considered more appropriate to the occasion than the scientific progress which undoubtedly forms the most striking characteristic of the century which we have just left behind us. The extraordinary, almost miraculous spread of the spirit of scientific inquiry, and the consequent rapid advance of science, are not only unparalleled in the world's history, but constitute a development which nobody at the commencement of the 19th century would probably have been bold enough to predict. We have been advancing by leaps and bounds, until at the beginning of the new century we find ourselves in the possession of knowledge, and means of applying that knowledge to practical en:ls, which far transcend the wildest dreams of enthusiasts in bygone ages.

It was therefore a peculiarly happy suggestion on the part of the organisation committee of the first International Physical Congress held at Paris in connexion with the 1900 Exhibition, that the occasion of its meeting should be rendered memorable by the publication of a work of more than passing interest-a sort of résumé of the state of physical knowledge up to the end of the nineteenth century. In order to obtain trustworthy and authoritative accounts, the committee invited the co-operation of a number of eminent specialists in various branches of physics. The results of this effort are embodied in the three volumes before us, and it must be confessed that, so far as physical science is concerned, no more valuable or interesting mode of commemorating the close of the nineteenth century could have been found.

Vol. I. deals with General and Molecular Physics, and opens with a most interesting paper by M. H. Poincaré" On the Relations between Experimental and Mathematical Physics." The subjects which are discussed in this volume are: standards of length, national physical laboratories, thermometric scales, pyrometry, dynamical equivalent of heat, specific heat of water, velocity of sound, Bjerknes's theory of hydrodynamic actions-at-a-distance,

elasticity of crystals, deformation of solids, constitution of alloys, properties of solids under pressure, fusion and crystallization, rigidity of liquids, capillary phenomena, diffusion of gases, osmosis, kinetic theory of gases, critical constants and specific heats of gases.

Vol. II., which opens with a paper by Lord Kelvin "On the Motion of an Elastic Solid traversed by a Body Acting on it by Attraction or Repulsion," is a collection of memoirs on Light, Electricity, and Magnetism, and contains papers on radiation, dispersion, velocity of light and electromagnetic waves, mode of energy propagation in the electromagnetic field, Hertzian waves, coherers, electrolytic dissociation, standards of E.M.F., electrochemical equivalents of silver, copper, and hydrogen, hysteresis, magnetostriction, and physical changes due to magnetization.

Vol. III. is devoted to physical problems of comparatively recent origin, and deals with a wide range of subjects of surpassing interest, among which may be mentioned recent theories of magneto-optic phenomena, theory of dispersion in metals, radioactive substances, cathode-rays, J. J. Thomson's suggestions regarding the constitution of matter, counter-E.M.F. of electric arc, polyphase currents, oscillographs, constant of gravitation, glaciers, atmospheric electricity, solar physics, biological physics.

A mere enumeration of the subjects dealt with is sufficient to show the importance of the work under review; and since all the papers have been contributed by specialists of acknowledged eminence, the work is one which probably few students of physics would care to be without. As a work of reference it is bound to find a place in every physical library worthy of the name.

XXXII. Proceedings of Learned Societies.

GEOLOGICAL SOCIETY.

[Continued from p. 160.]

April 24th, 1901.-J. J. H. Teall, Esq., M.A., F.R.S.,
President, in the Chair.

THE following communications were read :

1. Notes on two Well-Sections.' By the Rev. R. Ashington Bullen, B.A., F.L.S., F.G.S.

The well-section at Southwark passes through sand and gravel, etc. 34 feet, London Clay 75 feet, Woolwich and Reading Beds 56 feet 9 inches and Thanet Sand 36 feet 6 inches, into Chalk which was bored to a depth of 148 feet.

The well-section at Dallinghoo Post-Office, near Wickham Market (Suffolk), penetrated 53 feet of blue Chalky Boulder-clay, into 20 feet of Sand and Gravel, water being found at a depth of 79 feet. Liassic and Oxford Clay fossils were found in the Boulderclay and stones, one of which is considered by Prof. T. Rupert Jones

to have probably come from the Carboniferous rocks and one from the Bunter. The Sands contain no Crag fossils. Mr. F. Chapman, A.L.S., determined fossils from some of the boulders, from the fragments of stone found in the Sands, and from the Sands themselves. The last consist of Cretaceous foraminifera.

2. On the Geological and Physical Development of Antigua.' By Prof. J. W. Spencer, Ph.D., M.A., F.G.S.

Antigua and Barbuda rise from the bank which occupies the north-eastern portion of the chain of the Lesser Antilles. The part of the bank on which these two islands are founded is submerged to the very uniform depth of about 100 feet, but from other islandgroups it is separated by depressions of 1800 to 2500 feet. The margins of the bank are abrupt and precipitous, and are indented by deep valleys extending to the more profound depressions. The igneous basement-rocks of the island form the south-western mountain-belt. They are porphyritic andesites or porphyrites, with breccias and ashes which dip north-eastward. Associated with these rocks, and probably overlying them, are limestones which have not yet yielded fossils. The second and median belt of the island consists of stratified tuffs, with included marine and freshwater cherts. From the evidence of fossils these rocks may be Upper Eocene or Lower Miocene, and they manifestly are closely related to the rocks which follow them. The succeeding formation consists of earthy marls associated with beds of white limestone, and is apparently conformable to the underlying tuffs. A list of fossils is given, from which it is concluded that the beds are of Upper Oligocene age. Next follows a creamy-white, calcareous sandstone, and then the Friar's Hill Series of conglomerates and marls, resting unconformably on the white limestones, and considered to be of late Pliocene or early Pleistocene age. These are succeeded by the Cassada Garden Gravels, recent marls containing land-shells some of which are extinct, and coral-reefs, none of which are raised.

This

An account of the erosion-features of the region is given, and from this the following conclusions are drawn :--The region was an extensive land-surface, probably at least 2000 feet higher than now, during the Mio-Pliocene period, and was reduced by denudation to a comparatively low elevation before the close of that time. was followed by a submergence (the Friar's Hill) to a depth of 200 feet below the present altitude. At the close of the Pliocene period there was another elevation to an extent probably exceeding 3000 feet, as shown by the channels on the submarine plateau between Antigua and Guadeloupe. This did not continue sufficiently long to complete the dissection of the tablelands, and consequently the Antigua-Barbuda mass remains intact. Then followed a subsidence culminating in a 75-foot submergence, a re-elevation to 100 feet above the present level, when the shallow channels in the submarine bank were formed, and possibly one or two other small movements.