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depth in the solar atmosphere through which the radiation would have to pass. On the other hand, if the spot was floating above the absorbing atmosphere the radiation from it would remain constant in any position on the solar disc.

The following is the value of the heat radiation from the photosphere taken along a radius of the sun, where 0 = centre and 100 the limb. The radiation R equals 100 at the centre.*

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It will be seen by the following observations of spots, taken from August 5 to November 9, that there is distinct evidence that the radiation from the spot does not fall off as rapidly when near the limb as the neighbouring photosphere; in fact, the ratio u/C remains nearly constant, whereas the ratio u/N gets nearer unity as the spot approaches the limb. The spot observed on the 22nd of October is a good example, as the same spot was observed again on the 26th, 29th, and on the 30th, when it had reached within a distance, D, of 95 from the centre. It will be seen that on these four dates the ratio u/C was respectively 0.338, 0-360, 0:313, 0.356, whereas the ratio u/N was 0:349, 0:410, 0:706, 0.783.

Langley,t in 1874 and 1875, measured the radiation from san spots. He used a thermo-pile and galvanometer, and obtained as the mean of his results a ratio of 0.54 +0:05.

His method was first to take a reading in the neighbourhood of the spot, but between it and the centre of the disc. He then took a reading in the umbra, and, finally, a third reading in the neighbourhood between the spot and the edge of the sun.

* “The Absorption of Heat in the Solar Atmosphere," by W. E. Wilson and A. A. Rambaut, 'Proceedings of the Royal Irish Academy,' 3rd series, vol. 2, No. 2.

+ 'Monthly Notices,' vol. 37, No 1.

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The mean of the two photospheric readings he used as a divisor for the umbral reading. He then says, “ The decrement of heat as we approach the limb is, though not exactly, yet so very nearly, in the same ratio for photosphere and spots, that no correction is needed on this account for the present observations."

If Langley failed, through want of instrumental means, to notice the difference between the absorption in a spot and the photosphere near the limb, his method would make his umbral readings too high. The mean of twenty observations here equals 0-356, against Langley's 0:54. This is a serious difference, and, I think, can only be accounted for either by the use of superior instrumental means, or by a possible variation in the radiation of spots in different years of the sun spot cycle.

It is difficult to see how too low a value for umbral radiation could be got, whereas too high a one might be found by want of definition and trembling in the image, so that some of the penumbral radiation would reach the thermo-couple.

II. “ Experimental Investigations on the Effective Temperature

of the Sun, made at Daramona, Streete, Co. Westmeath." By W. E. Wilson, M.R.I.A., and P. L. GRAY, B.Sc., Assoc. R.C.S., Demonstrator of Physics, Mason College, Birmingham. Communicated by G. JOHNSTONE STONEY, F.R.S. Received January 4, 1894.

(Abstract.) The only tolerably complete series of investigations on this subject up to the present time have been those of Rossetti and Le Châtelier. The results given by other writers have depended more or less on guesses relative to the law connecting radiation and temperature, and differences on this point alone have given values varying between 1500° and 3,000,000° to 5,000,000° C.

Rossetti worked with a thermo-pile, exposed directly to the heat of the sun; the law connecting the deflections of the galvanometer with the temperature of an artificial source of heat having been obtained up to a temperature of about 2000° C., from the deflection produced by the heat of ihe sun the solar temperature was calculated by extrapolation.

Le Châtelier worked on an entirely different principle, measuring the intensity of the light transmitted through a certain piece of red glass, first from sources at known temperatures up to 1700° or 1800', and, secondly, from the sun, the temperature of which was then obtained, as in Rossetti's case, by a process of extra-polation, which is, of course, necessary in any method, until we can raise substances to a temperature actually as high as that of the sun, an experiment at present impossible.

Rossetti obtained finally a temperature of 10,000° C., approximately, while Le Châtelier gives 7600° (+1000°) as the mean of his results. In the paper the difference between Rossetti's result and our own (6200° C.) is discussed, and a possible explanation given.

The method adopted by the authors is a zero method, and the essential point is the balancing of the heat from the sun with that from a platinum strip heated to a high known temperature.

The artificial source of heat was a modified forın of Joly's “meldometer," the calibration of which can be performed with a very high degree of accuracy. The “radiation balance" is a form of Boys's radio-micrometer containing a duplex circuit, so designed that the heat from the sun can be made to exert a turning moment in the opposite direction to that due to the artificial source of heat, and by making the apparent area of the latter sufficiently great, the radiation from it may be increased so far as to equal that arriving at the radio-micrometer from the sun.

The following points are considered, after descriptions of the method and apparatus have been given :1. The law connecting radiation and temperature.

This is probably the most important factor in the value of the final result. Numerous investigations on the point have been made, which are referred to in the paper.

After a careful series of experiments we have come to the conclusion that (at least for bright platinum) Stefan's "law of the 4th power” holds, * i.e., that for high temperatures (say over 600° or 700° C.) if R = the radiation from a source whose absolute temperature is T, then

RO T. 2. The emissive power of platinum at high temperatures compared

with that of lamp-black.

On this point the value obtained by Rossetti was used,

some considerations being given in support of his figures. 3. The amount of the atmospheric absorption.

This is fully discussed, and again the value obtained by Rossetti is used.

Langley's theoretical value for percentage absorption of radiation from a body in the zenith, viz., 41 per cent., is shown to be possibly too great; Rossetti obtained 29 per cent., which appears to be the value best supported by experiment.

The climate of Ireland entirely prevents a systematic series

of investigations on this particular point. Several subsidiary questions are also discussed, and, finally, the results of about sixty-nine observations are given, which lead to a final mean result for the effective solar temperature of 6200° C.

It is pointed out, in conclusion, that the method would probably give excellent results if adopted in some country in or near the tropics, where atmospheric conditions can be trusted to remain more constant for some weeks, or even days, together, and where a series of observations taken at the same part of the year throughout the period of a sun-spot cycle might be hoped to settle the question of how (or if) the solar temperature varies during this time, as any error in the absolute value obtained may probably be considered constant, so that comparative values from year to year might be trusted to indicate any change.

Stefan, however, stated the law as applying to the "pure " radiation of a surface of perfect emissive power.

III. “ Experiments on a Fundamental Question in Electro

Optics : Reduction of Relative Retardations to Absolute."

By JOHN KERR, LL.D., F.R.S. Received March 9, 1894. To prepare the way, I begin by recalling these well-known facts : that when light passes through an electrostatically strained medium in a direction perpendicular to the line of electric force, it undergoes a uni-axal double refraction, the optic axis coinciding with the line of force; that with reference to this action, dielectrics are divisible into two classes, the positive* and the negative, f which are optically related to each other in the same way as the positive class of crystals to the negative; that the intensity of the action, or the quantity of optical effect per unit thickness of the dielectric, is measured by the product CF, where C is a constant which is characteristic of the medium, and P is the value of the resultant electric force : that the effects are generally observed and examined still as they were discovered first, by simple experiments with a pair of Nicol's prisms and a slip of strained glass or other phase-difference compensator.

In every such experiment the effect specified by the compensator is a difference of phases, or a relative retardation; and we may therefore view it as a resultant effect —that is to say, as the resaltant, or the difference, of electrically-generated absolute retardations of two component lights, whose planes of polarisation are parallel and perpendicular to the line of electric force. What, then, are the values of these two absolute retardations in any given case ? What are the two absolute components of any electrically-generated relative retardation? Such is the question here proposed for solution by experiment.

As long ago as 1882, and several years following, I was much occupied at intervals with this interesting question. In the summer of 1885, in some experiments with the dielectric CS, I obtained results as decisive as could be desired. Other dielectrics, both solid and liquid, were tried afterwards, but only with partial success, the experimental difficulties being, in some cases, too much for my methods and time. To these cases I shall make no further reference, as I will keep to the one line of experiment, and to those experiments in particular in which the indications were quite regular and unmis. takeable. With these limitations, the inductions extend to four liquid dielectrics, two positive and two negative; and all the experiments point clearly in one direction. General Result.-It appears that the proper and immediate optical

* Corbon disulphide, the hydrocarbons, &c. + Amyl oxide, the heavy oils, &c.

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