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due to the dense core of the arc is found to consist of all the lines which this part of the arc alone can give, the longer lines given by the outer less dense strata extending beyond the spectrum of the core to the various distances from the centre to which the vapour capable of giving them extends.

The lines thus photographed are pointed at either end, and disappear from the centre in the order of their lengths, so that a double or duplicate determination, exquisitely symmetrical, of length is thus obtained.

Although the lengths, thicknesses, and intensities of the lines are thus readily recorded, we have so far no scale by which to fix their positions. In order to obviate this objection, the solar spectrum is photographed on each plate immediately above or below the metallic spectrum under examination.

This new method has enabled me to register no less than four different spectra on the same wet plate. Those familiar with photographic processes will immediately see how it is that the number is not forty instead of four.

§ 9. Results of these Arrangements.

We have thus an absolute comparison rendered possible, by means of photography, between the lines of the spectrum of a metal, and the lines of the spectrum of the sun. Plate IV. gives an idea of such photographs. In the case of most of the thick lines we get a thick line in the solar spectrum corresponding with the

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lines of the metal. In some cases the lines of the metal are of different lengths. The reason of that is, that care has been taken to photograph on the plate the lines due to the various strata of the vapour, from the rarest vapour, which is obtained at the outside of the electric arc, to the densest, which occupies the centre of the core. We thus get a most beautiful gradation, as we pass from the outside part of the spectrum to the inside. The inside part represents the complete spectrum of the core, and the outside the incomplete and almost monochromatic spectrum of the vapour which surrounds the denser core in the middle of the spark; thus we practically reduce the spectrum of the metal to one line.

§ 10. The Solar Spectrum.

We may next consider the application of spectrum photography no longer to the mere solar spectrum, but to the physics of the sun. What is the solar spectrum? It is the continuous spectrum of the sun, minus certain portions where the light of the continuous spectrum has been absorbed. What have been the absorbers? The gases and vapours, generally speaking, in an excessively limited zone of the sun's atmosphere, lying close to the bright sun we see-that is close to the photosphere. This zone is called the reversing layer. Then if the solar spectrum is the result of the absorption of this reversing layer, what will happen to the solar spectrum if the constitution of the layer changes? Obviously a

change in the solar spectrum. Now, the researches referred to in Chapter VIII., researches carried on by means of photography-show us that if we take any particular vapour in the reversing layer, which we may call A, for instance, and then assume that the quantity of A in the layer is reduced, the absorption of that particular vapour will be reduced; what then will be the result on the photograph of the solar spectrum? Some of the lines will disappear. Suppose that this particular vapour which we called A, instead of being assumed to decrease in quantity, increases in quantity, what will happen to the solar spectrum? The same researches tell us that as its quantity increases its absorption will increase, and that its increased absorption will be indicated by an increase in the number and in the breadth of the lines absorbed. What, then, will happen to the solar spectrum if any change of this kind is going on? The photograph of a solar spectrum taken, say, to-day, may be different from the photograph of the same part of the spectrum taken at some distant period. What is the distant period we do not yet know-whether three months, six months, six years, or eleven years, or a multiple of eleven years-but, at all events, there is reason to think already that if we had a series of photographs of the solar spectrum taken year by year, we should see changes in the spectrum. A photograph of a very limited portion of the solar spectrum will prove my case; and I could not have proved it if photography had not been called in, because if the existence of any particular metal, or of the increase of any particular

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