CHAP. XIII. Outer boundary of this hot region there appears to be a shell of excessively faint cloud, part of which is to be seen in Mr. De la Rue's photographs of the eclipse of 1860. It probably extends the whole way round the sun; it is, therefore, very desirable that this faint shell, which seems to lie at a distance of eight or ten seconds of space from the edge of the sun's disc, should be observed.”

Mr. Stoney was led to the opinion that in the upper regions of the sun's sodium atmosphere, at which the reversal of the lines takes place, the temperature is lower than that of a Bunsen's burner. The winged appearance of some lines, especially those of hydrogen, led Mr. Stoney to associate that appearance with a great quantity of the substance to which the lines were due; the absence of wings, as in the case of sodium, indicating a small quanAbsence of tity. The absence of nitrogen and oxygen from the sun. led to the conclusion that compound bodies exist in the The reason they are not revealed in the spectrum, being that the masses of the molecules are too high to enable them to reach the cool parts of the sun's atmosphere.*

nitrogen

and

oxygen.

sun.

Here then ends the long parenthesis, which has extended over five Chapters, in which I have attempted to show how Spectrum Analysis helps us in the study of Solar Physics. So far we have confined ourselves to the method employed by Kirchhoff which deals with the average spectrum of the sun. The remaining part of this book will deal in the main with the results obtained up to the present time by the new method, one in which each minutest portion of the sun is examined separately. To this new method reference has already been made in Chap. viii.

THE FIRST RESULTS OF THE NEW

METHOD.1

IN the year 1865 two very important memoirs, dealing CHAP.XIV with all the telescopic and photographic observations. accumulated up to that time on the subject of solar physics, were given to the world. One of them was privately printed in this country; the other appeared in the Comptes Rendus of the Paris Academy of Sciences.

I shall not detain you with a lengthened notice of these remarkable papers. I shall merely refer to the explanation Explanagiven in both of them of the reason that a sun-spot appears dark-the very keystone of any hypothesis dealing with the physical constitution of the sun.

English science, represented by Messrs. De la Rue, Stewart, and Loewy, said that a spot is dark because the solar light is absorbed by a cool, non-luminous, absorbing atmosphere, pouring down there on to the visible surface of the sun-in other words, on to the photosphere.

French science, represented by M. Faye, said that a spot is dark because it is a hole in the photosphere, and the feebly luminous and therefore radiating interior gases of the sun are there alone visible.

Now most of you will see in a moment that here was a clear issue, which probably the spectroscope, and possibly

A lecture delivered at the Royal Institution of Great Britain, Friday evening, May 28th, 1869.

P

tion of sun-spots.

CHAP. XIV. nothing else, could solve; for the spectroscope is an instrument whose special métier it is to deal with radiation and absorption. It tells us that the light radiated from different bodies gives us spectra of different kinds, according to the nature of the radiating body,-continuous spectra without bright lines in the case of solids and liquids; and bright lines, with or without continuous spectra, in the case of gases and vapours. It tells us also that absorption dims the spectrum throughout its length when the absorption is General general, and dims it here and there only when the absorpand selection is selective; the well-known Fraunhofer lines being, as tive absorp tion. you will readily see, an instance of the latter kind. So that we have general and selective radiation, and general and selective absorption.

Now then, with regard to the English theory, if there were more absorption in a spot than elsewhere, we might expect evidences of absorption; that is, the whole solar spectrum would be visible in the spectrum of a spot, but it would be dimmed, either throughout the length of the spectrum or in places only.

With regard to the French theory, having only radiating gaseous matter to deal with, we should, according to the then generally received idea, get bright lines only in the spot spectrum.

Here, then, was a tempting opportunity, and one which I considered myself free to use; for, although the spectroscope had then been employed-and you all know how nobly employed-for four years in culling secrets from stars and nebulæ, there was not, so far as I know, either published or unpublished observation on the sun, the nearest star to us. The field was therefore open for me, and I was not entering into another man's labour, when, on the 4th of March, 1866, I attached a small spectroscope to my telescope, in order to put the rival theories to a test, and thus bring another power to bear on a question which had remained a puzzle since it was first started by Galileo some two and a half centuries ago.