case of reflection from iron, as in most cases of transmission through salts of iron, the plane of polarization is turned in a direction contrary to that of the magnetizing current.

A Description of Spottiswoode's Pocket Polarizing Apparatus. By W. LADD.

On a Phenomenon of Metallic Reflection. By Professor G. G. STOKES, F.R.S.

The phenomenon which I am about to describe was observed by me many years ago, and may not improbably have been seen by others; but as I have never seen any notice of it, and it is in some respects very remarkable, I think that a description of it will not be unacceptable.

When Newton's rings are formed between a lens and a plate of metal, and are viewed by light polarized perpendicularly to the plane of incidence, we know that, as the angle of incidence is increased, the rings, which are at first darkcentred, disappear on passing the polarizing angle of the glass, and then reappear white-centred, in which state they remain up to a grazing incidence, when they can no longer be followed. At a high incidence the first dark ring is much the most conspicuous of the series.

To follow the rings beyond the limit of total internal reflection we must employ a prism. When the rings formed between glass and glass are viewed in this way, we know that as the angle of incidence is increased the rings one by one open out, uniting with bands of the same respective orders which are seen beneath the limit of total internal reflection; the limit or boundary between total and partial reflection passes down beneath the point of contact, and the central dark spot is left isolated in a bright field.

Now when the rings are formed between a prism with a slightly convex base and a plate of silver, and the angle of incidence is increased so as to pass the critical angle, if common light be used, in lieu of a simple spot we have a ring, which becomes more conspicuous at a certain angle of incidence well beyond the critical angle, after which it rapidly contracts and passes into a spot.

As thus viewed the ring is, however, somewhat confused. To study the phenomenon in its purity we must employ polarized light, or, which is more convenient, analyze the reflected light by means of a Nicol's prism.

When viewed by light polarized in the plane of incidence, the rings show nothing remarkable. They are naturally weaker than with glass, as the interfering streams are so unequal in intensity. They are black-centred throughout, and, as with glass, they open out one after another on approaching the limit of total reflection and disappear, leaving the central spot isolated in the bright field beyond the limit. The spot appears to be notably smaller than with glass under like conditions.

With light polarized perpendicularly to the plane of incidence, the rings pass from dark-centred to bright-centred on passing the polarizing angle of the glass, and open out as they approach the limit of total reflection. The last dark ring to disappear is not, however, the first, but the second. The first, corresponding in order to the first bright ring within the polarizing angle of the glass, remains isolated in the bright field, enclosing a relatively, though not absolutely, bright spot. At the centre of the spot the glass and metal are in optical contact, and the reflection takes place accordingly and is not total. The dark ring, too, is not absolutely black. As the angle of internal incidence increases by a few degrees, the dark ring undergoes a rapid and remarkable change. Its intensity increases till (in the case of silver) the ring becomes sensibly black; then it rapidly contracts, squeezing out, as it were, the bright central spot, and forming itself a dark spot, larger than with glass, isolated in the bright field. When at its best it is distinctly seen to be fringed with colour, blue outside, red inside (especially the former), showing that the scale of the ring depends on the wave-length, being greater for the less refrangible colours. This rapid alteration taking place well beyond the critical angle is very remarkable. Clearly there is a rapid change in the reflective properties of the metal, which takes place, so to speak, in passing through a certain angle determined by a sine greater than unity.

1876.

4.

I have described the phenomenon with silver, which shows it best; but speculummetal, gold, and copper show it very well, while with steel it is far less conspicuous. When the coloured metals gold and copper are examined by the light of a pure spectrum, the ring is seen to be better formed in the less than in the more refrangible colours, being more intense when at its best; while with silver and speculummetal there is little difference, except as to size, in the different colours. Hæmatite and iron pyrites, which approach the metals in opacity and in the change of phase which they produce by reflection of light polarized parallel relatively to light polarized perpendicularly to the plane of incidence, do not exactly form a ring isolated in a bright field; but the spot seen with light polarized perpendicularly to the plane of incidence is abnormally broad just about the limit of total reflection, and rapidly contracts on increasing the angle of incidence.

It seemed to me that a sequence may be traced from the rapidly contracting rings of diamond seen in passing the polarizing angle of that substance, through the abnormally broad and rapidly contracting spot seen with iron pyrites just about the limit of total reflection, and the somewhat inconspicuous ring of steel seen a little beyond the limit, to the intense rapidly contracting ring of silver seen considerably beyond the limit. If so, the full theory of the ring will not be contained in the usually accepted formulæ for metallic reflection, modified, as in the case of transparent substances, in accordance with the circumstance that the incidence on the first surface of the plate of air is beyond that of total reflection.

MacCullagh was the first to obtain the formula for metallic reflection, showing that they were to be deduced from Fresnel's formulæ by making the refractive index a mixed imaginary, though they are usually attributed to Cauchy, who has given formulæ differing from those of MacCullagh merely in algebraic detail. As regards theory, Cauchy made an important advance on what MacCullagh had done in connecting the peculiar optical properties of metals with their intense absorbing power. Now Fresnel's formulæ do not include the phenomena discovered by Sir George Airy, which are seen in passing the polarizing angle of diamond, and which have been more recently extended by M. Jamin to the generality of transparent substances; and if these pass by regular sequence to those I have described as seen with metals beyond the limit of total internal reflection, it follows that the latter would not be completely embraced in the application of Fresnel's formulæ, modified to suit an intensely absorbing substance and an angle of incidence given by a sine greater than unity†.

ELECTRICITY.

On the Contact Theory of Voltaic Action. By Professors AYRTON and Perry.

On a new Form of Electrometer. By Prof. J. DEWAR, F.R.S.E.

On a Mechanical Illustration of Electric Induction and Conduction.
By OLIVER J. LODGE, B.Sc.

The paper describes the construction of a model which illustrates Prof. Clerk Maxwell's theory of electric action on the hypothesis of stress in a dielectric me

*The apparent difference between MacCullagh and Cauchy as to the values of the refractive indices of metals is merely a question of arbitrary nomenclature.

It was long ago observed, both by Professor MacCullagh and Dr. Lloyd, that when Newton's rings are formed between a glass lens and a metallic plate, the first dark ring surrounding the central spot, which is comparatively bright, remains constantly of the same size at high incidences, although the other rings, like Newton's rings formed between two glass lenses, dilate greatly as the incidence becomes more oblique. See Proceedings of the Royal Irish Academy,' vol. i. p. 6.

dium, and which consists essentially of an endless cord passing with friction through buttons supported on elastic strings. By altering the relation between the friction and the elasticity of different parts, it can be made to exhibit very completely the phenomena observed when an electromotive force is made to act:-(1) between the ends of a metal wire; (2) through an electrolytic liquid, when it illustrates the convection of electricity by the cathion and the polarization of the electrode; (3) in an accumulator with perfectly insulating dielectric, when it shows the polarization of the dielectric, the displacement of electricity in the direction of the force, the tension along the lines of force, occasional possible disruptive discharge, and consequent possible internal charge; (4) across a dielectric which is homogeneous, but has a slight conducting power, showing in this case a continuous ordinary conduction-current, in addition to the variations of electric displacement; (5) across a non-homogeneous or stratified dielectric, in which a "residual charge is possible. If made of proper materials, the model would exhibit this residual charge quantitatively as well as qualitatively; and, in fact, the investigation "On the Theory of a Composite Dielectric" (in arts. 328-330 of Maxwell's Electricity') would apply to it with little modification. It further illustrates incidentally the action of a voltaic cell and of a submarine cable.

·

On a Mechanical Illustration of Thermoelectric Phenomena.
By OLIVER J. LODGE, B.Sc.

The model which illustrates metallic conduction in the preceding communication is supposed to be modified, so that all the buttons execute very rapid isochronous simple harmonic motions, sliding to and fro on the cord. The rate of cooling of a body placed in an enclosure at absolute zero is then seen to be proportional to the absolute temperature of the body, and to depend on its specific electrical resistance. The electric condition of tourmaline is explained by an hypothesis as to the nature of its internal structure; and the amount of heat generated by an electric current passing through a metallic conductor is deduced in accordance with Joule's law. An hypothesis is then started as to the nature of the internal actions at a junction either of two different metals at the same temperature or of two parts of the same metal at different temperatures; and, on the strength of this hypothesis, electromotive force produced by contact, the Peltier effect, and Thomson's electric convection of heat are all illustrated. The exact laws which have been experimentally established for these effects may possibly be deducible from considerations founded on the model; but this has not yet been properly done*.

On the Protection of Buildings from Lightning.

By Professor J. CLERK MAXWELL, F.R.S.

Most of those who have given directions for the construction of lightningconductors have paid great attention to the upper and lower extremities of the conductor. They recommend that the upper extremity of the conductor should extend somewhat above the highest part of the building to be protected, and that it should terminate in a sharp point, and that the lower extremity should be carried as far as possible into the conducting strata of the ground, so as to "make what telegraph engineers call "a good earth."

The electrical effect of such an arrangement is to tap, as it were, the gathering charge, by facilitating a quiet discharge between the atmospheric accumulation and the earth. The erection of the conductor will cause a somewhat greater number of discharges to occur at the place than would have occurred if it had not been erected, but each of these discharges will be smaller than those which would have occurred without the conductor. It is probable, also, that fewer discharges will occur in the region surrounding the conductor. It appears to me that these arrangements are calculated rather for the benefit of the surrounding country, and for the *These two papers are published, with some additions, in the Phil. Mag. ser. 5, vol. ii. pp. 353 and 524.

relief of clouds labouring under an accumulation of electricity, than for the protection of the building on which the conductor is erected.

What we really wish is to prevent the possibility of an electric discharge taking place within a certain region, say, the inside of a gunpowder manufactory.

If this is clearly laid down as our object, the method of securing it is equally clear.

An electric discharge cannot occur between two bodies unless the difference of their potentials is sufficiently great compared with the distance between them. If, therefore, we can keep the potentials of all bodies within a certain region equal or nearly equal, no discharge will take place between them. We may secure this by connecting all these bodies by means of good conductors, such as copper-wire ropes; but it is not necessary to do so; for it may be shown by experiment that if every part of the surface surrounding a certain region is at the same potential, every point within that region must be at the same potential, provided no charged body is placed within the region.

It would therefore be sufficient to surround our powder-mill with a conducting material (to sheathe its roof, walls, and ground-floor with thick sheet-copper), and then no electrical effect could occur within it on account of any thunder-storm outside.

There would be no need of any earth-connexion. We might even place a layer of asphalt between the copper floor and the ground, so as to insulate the building. If the mill were then struck with lightning, it would remain charged for some time, and a person standing on the ground outside and touching the wall might receive a shock; but no electrical effect would be perceived inside, even on the most delicate electrometer. The potential of every thing inside, with respect to the earth, would be suddenly raised or lowered, as the case might be; but electric potential is not a physical condition, but only a mathematical conception, so that no physical effect could be perceived.

It is therefore not necessary to connect large masses of metal, such as engines, tanks, &c., to the walls, if they are entirely within the building.

If, however, any conductor, such as a telegraph-wire or a metallic supply-pipe for water or gas, comes into the building from without, the potential of this conductor may be different from that of the building, unless it is connected with the conducting shell of the building. Hence the water or gas supply-pipes, if any enter the building, must be connected to the system of lightning-conductors; and since to connect a telegraph-wire with the conductor would render the telegraph useless, no telegraph from without should be allowed to enter a powder-mill, though there may be electric bells and other telegraphic apparatus entirely within the building.

I have supposed the powder-mill to be entirely sheathed in thick sheet-copper. This, however, is by no means necessary in order to prevent any sensible electric effect taking place within it, supposing it struck by lightning. It is quite sufficient to enclose the building with a network of a good conducting substance. For instance, if a copper wire, say No. 4, B.W.G. (0-238 inch in diameter), were carried round the foundation of a house, up each of the corners and gables, and along the ridges, this would probably be a sufficient protection for an ordinary building against any thunder-storm in this climate. The copper wire may be built into the wall to prevent theft, but it should be connected to any outside metal, such as lead or zinc on the roof, and to metal rain-water pipes.

In the case of a powder-mill, it might be advisable to make the network closer by carrying one or two additional wires over the roof and down the walls to the wire at the foundation. If there are water- or gas-pipes which enter the building from without, these must be connected with the system of conducting-wires; but if there are no such metallic connexions with distant points, it is not necessary to take any pains to facilitate the escape of the electricity into the earth.

It is desirable, however, to provide for the safety not only of the building itself, but of the system of conductors which protects it. The only parts of this system which are in any danger are the points where the electricity enters and leaves it. If, therefore, the system terminates above in a tall rod with a sharp point, and downwards in an "earth wire," the external discharge will be almost certain to occur at the ends of these electrodes, and the only possible damage will be the loss

of a few particles from their extremities; but even if the rod and wire were destroyed altogether, the building would still be safe.

On Compass Correction in Iron Ships. By Sir W. THOMSON, D.C.L., F.R.S.

Effects of Stress on the Magnetization of Iron.

By Sir W. THOMSON, D.C.L., F.R.S.

On Contact Electricity. By Sir W. THOMSON, D.C.L., F.R.S.

ACOUSTICS.

On the Conditions of the Transformation of Pendulum-Vibrations; with an experimental illustration. By R. H. M. BOSANQUET, Fellow of St. John's College, Oxford.

Under certain circumstances, a pendulum-vibration of given period can give rise to impulses which support vibrations whose periods are,,,..... of the period of the original vibration. The conditions under which this takes place are of interest.

Wheatstone enunciated the following as an experimental law :-A periodic impulse can sustain vibrations whose frequencies are multiples of that of the impulse. This was supported by an experiment in which the harmonics of a Jew's harp are obtained from it by an adjustable resonator. But a general law cannot be proved by a particular experiment.

An experiment was adduced in contradiction of the generality of the above law. It can be shown that the stopped pipes of the organ are incapable of supporting the vibration of resonators tuned to their octave and double octave, while open pipes are capable of doing so.

As the result of mechanical theory, the law may be enunciated that no pendulumvibration can be maintained in a vibrating system, unless the acting forces contain impulses of the same period as the vibration maintained.

The experiments commonly shown, in which a simple pendulum-vibration is made to support its harmonics, generally depend on a transformation of the vibration in the transmission of the impulse. The apparatus exhibited forms a type of the general process of transformation by transmission.

A metronome vibrating seconds furnishes the fundamental vibration: a number of small pendulums vibrate 2, 3, 4, 5, 6, 7, and 8 times in a second. By making connexions between the metronome and the pendulums with elastic cord in different ways, the different kinds of transmission (with and without transformation) can be illustrated.

When the cord is tight, the impulses are transmitted without transformation; when the cord goes slack during the vibration, the impulses are transformed into a series of pulls. In the first case the small pendulums are not affected; in the second they are generally set in vibration.

The following points are illustrated by the experiment with the partly slack cord, where the impulses constitute a series of pulls :

The common exposition of the theory of musical sounds, in which the impulses are compared to the blows of a hammer, really makes a very complex effect the basis of operations. The notes thus constructed differ from simple musical tones in having the power of supporting the vibrations of their harmonics.

The cases of the notes of the siren and the harmonium, in which the sound is produced by a series of jets of air, are illustrated by the same experiment.

An experiment of Prof. Mayer's, for the analysis of the sound of a reed pipe, by