present state of our knowledge-to estimate their probable effects.

I confess, therefore, that at this stage of my subject I am very far from sharing that confidence which I find some men possess in dealing with problems of solar physics. I shall not pretend to place all the phenomena in that due order in which they appear in the theories hitherto propounded. I can only look on with a sense of bewildered admiration while the professors of rival theories exhibit the physical habitudes of the Sun as obviously explicable according to contradictory hypotheses. I must admit that it seems to me that only a very energetic forgetfulness of a large proportion of the evidence can account for the adoption of these theories. I must content myself, therefore, with an exceedingly brief statement of certain general relations, which are all that I find satisfactorily exhibited by what has as yet been learned respecting the Sun.

We have in the Sun a vast agglomeration of the elements we are familiar with on Earth; and this vast agglomeration is subject to two giant influences, producing in some sort opposing effects-viz., a temperature far surpassing any we can form any conceptions of, and a pressure (throughout nearly the whole extent of the solar globe) which is perhaps even more disproportionate to the phenomena of our experience. Each known element would (beyond all question) be vaporised by the solar temperature at known pressures; each would (there can be little question) be solidified by the vast solar pressures, did these occur at

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known temperatures. Now, whether under these circumstances the laws of gaseous diffusion prevail where the elements are gaseous in the solar globe; whether where liquid matter exists it is in general bounded in a definite manner from the neighbouring gaseous matter; whether any elements at all are solid, and if so under what conditions their solidity is maintained and the limits of the solid matter defined-all these questions are such as we must answer before we can form a satisfactory view of the solar constitution; and yet they are questions which we have at present no means of answering. Again, we must learn how far combustion, properly so called, can take place within the Sun's mass, and whether those processes which we recognise as combustion are the only processes of combustion which can actually take place there. For aught that is yet known, the intensity of the forces at work upon and within the Sun may wholly prevent the occurrence of any processes of combustion familiar to ourselves; while other processes of true combustion altogether unthought of by us may be in continual action.

Assuming, however, that some general resemblance exists between the processes at work upon the Sun and those we are acquainted with (the wildest assumption possible), we may imagine that the various elements in the solar substance ordinarily exist down to certain definite levels in the gaseous form, at lower levels (definite for each) in the liquid form, at yet lower levels in the solid form. That part of each element which is gaseous must again be divided into two portions-that whose light is capable of giving charac

teristic spectra of lines or bands (these spectra being, however, different for portions lying at different depths), and that lower portion whose light is capable of giving a continuous spectrum.

Now, here we approach the great difficulty of interpreting the results of the spectroscopic analysis of the Sun. We have no means of learning whence that part of the light comes which gives the continuous spectrum. When we recognise certain dark lines, we know certainly that the corresponding element exists in the gaseous form at a lower temperature than the substance which gives the continuous spectrum. And so, also, we can interpret the appearance of bright lines. They show beyond question that the corresponding element exists in the gaseous form at a higher temperature than the substance which gives the continuous spectrum. But as regards that continuous spectrum itself we can form no such exact opinion. It must be remembered that a substance giving a continuous spectrum is not necessarily opaque to light from a substance at higher temperature also giving a continuous spectrum. It is capable of exercising a general absorption, but not necessarily (nor probably, under such conditions as exist in the Sun) of exercising an absorption at once general and complete. Hence we have no means of determining how great a depth of the solar substance is concerned in sending out the light which gives the continuous background of the spectrum. This light may come from the surface layers only-but it may be a shell whose thickness

forms no inconsiderable aliquot part of the Sun's diameter. And the reversal of the lines of certain elements, although it cannot take place at such excessive depths, may yet take place very far below the visible limits of the photosphere. For, as I have already shown, a depth of a few hundred miles would be wholly inappreciable in the most powerful telescope (spectroscopically armed), and so no peculiarities would be recognised as the result of processes taking place within such distances of the solar surface, however diligently the edge of the solar disc might be examined with the spectroscope. And, further, as respects the examination of the Sun's edge, on which so much stress has been laid, it is far from unlikely—if, indeed, it is not to be regarded as certain-that the visible edge of the solar disc lies considerably above the true limit of the photosphere. That light at the Sun's edge which seems to belong to the hemisphere of photospheric matter turned towards us comes probably from parts of the photosphere which lie really beyond the borders of that hemisphere, and are simply brought into view through the refractive power of the lower layers of the solar atmosphere. I am not here venturing a mere opinion or conjecture, though I profess no certainty of conviction in the matter. We have the evidence of

*This is in no way opposed to the evidence adduced by Sir John Herschel from the apparent uniformity of light derived from a glowing and transparent liquid not uniformly deep; for every part of such a liquid (at least in Sir John Herschel's experiment) is at appreciably the same temperature. If the lower layers could be heated to a higher temperature, the effect of depth would become apparent.

facts-observed by Carrington, Secchi, and others-showing that the motions of the spots across the solar disc are really affected, as respects apparent rate, by the refractive power of the solar atmosphere. And it is impossible to doubt that if the apparent place of a spot can be affected in this way, then the solar regions beyond that hemisphere which is turned towards us at the moment, must be brought into view and form the real limits of the solar disc.*

I am sensible that I am not making definite statements as to the Sun's condition, but only stating difficulties. The difficulties are real, however, not imaginary, and ignoring them can serve no useful purpose.

When we turn to the details of the solar orb and its surroundings, we find the evidence slightly more definite; but still great difficulties surround us. In the course of the several chapters bearing on these matters nearly all the known facts which bear directly on the views we are to form respecting the Sun's physical constitution have been discussed as they have been described, so that but few words about the several solar features are here called for.

Taking the actual telescopic aspect of the solar spots

This really amounts to saying that the Earth, if viewed from the solar photosphere, would be visible above the solar horizon (as our Sun is after real sunset and before real sunrise) when the geometrical line to her passed below that horizon. We cannot doubt that the solar atmosphere would exert this refractive effect at least as powerfully as our own; and if the Earth could be seen in this way from parts of the Sun really turned away from her, then certainly those parts of the Sun must be visible from the Earth; for a visual ray passes along the same path from whichever of its extremities we suppose it to travel.