LIST OF ILLUSTRATIONS.

A CHART OF MARS ON MERCATOR'S PROJECTION

FIG. 1. SOLAR CLOUD PROMINENCE SEEN BY PROF.

Frontispiece

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THE

ORBS AROUND US.

INTRODUCTION.

THE GAMUT OF LIGHT.

A FEW YEARS AGO I had occasion, during the course of a lecture on astronomy, to explain the nature of spectroscopic analysis to a mixed audience. I had gone through the usual statement of the laws on which this mode of research depends; but I felt convinced that the explanation had been insufficient. That sense of sympathy which enables every lecturer to tell whether his hearers are following him assured me that the audience, with all willingness to be convinced, had not grasped the essential principle on which spectroscopic analysis depends. It will be understood that my object was not to give a complete account of the new analysis; but it was essential that the convincing nature of the evidence which the analysis affords should be brought clearly before the

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audience. I knew that before long I should have to tell them certain very startling facts, and that, unless they had some solid ground for believing these facts, the whole lecture would resolve itself into a mere series of statements to be accepted on trust, whereas the end and aim of lecturing is, or should be, to demonstrate.

In this strait, it occurred to me to re-state, but in a new form, the evidence on which the application of the spectroscope depends. It is no new conception to compare light and sound together, or to illustrate the analysis of light by a reference to the combinations of musical notes. In fact, I had long before employed this method of illustrating the subject. But it was a new thing--to me at least-to test the efficiency of this method of explanation by bringing it before an audience immediately after the ordinary explanation had failed. It was, therefore, with no small interest and satisfaction that I found the audience grasping at once the points I was so anxious to enforce, and becoming eager to hear how the mode of analysis they now trusted in had been applied by physicists to astronomical problems.

This experience, and the fact that day after day new facts are being revealed by the spectroscope, induce me to think that an explanation of the powers of the instrument on the plan referred to may be serviceable to many who daily hear the work of the spectroscope mentioned, and have perhaps often seen its action scientifically explained, but have yet no

clear and definite ideas of the nature of the evidence it supplies, or of the reasons on which men of science base their acceptance of such evidence.

Every one is familiar with the gamut of sound. It is also easy to conceive the orderly succession of notes, separated by definite tone-intervals, replaced by an arrangement in which the difference between successive notes should be imperceptible. We can imagine, for instance, that in place of the white and black keys between two C's of a piano there might be an indefinite number of keys, so that, supposing these swept from C to C, every possible gradation of sound between those notes would become audible. We shall call this arrangement a continuous gamut.

Now it is found that when the light of an incandescent solid or fluid body is dispersed by a prism, it forms a rainbow-tinted streak, in which all orders of colour from red, through orange, yellow, green, blue, and indigo, to violet, are present, without break or interruption. So that we can compare this rainbowtinted streak (or spectrum, as it is called) with the stream of sound in which all orders of tone, from one C to the next above it, are heard without break or interruption. We need not concern ourselves about the scientific exactness of the illustration if it suffices for our purpose.1

It is the attempt to secure at the same time clearness of illustration and strict scientific exactness which causes so many explanations to perplex instead of edifying. Scientific exactness can come afterwards if the beginner is encouraged to pursue the devious tracks which lead to it, by obtaining a clear view of what he will gain by the labour.

And now, before proceeding, let us take an example of the application of this first and fundamental fact. With special exceptions, into the nature of which we need not now enter, it may be said that all incandescent solid and fluid bodies show this continuous rainbow-tinted streak, and that only the light from such bodies will exhibit a continuous streak of colour from deepest red to deepest violet. This is an experimental fact. Now suppose there is some selfluminous body that we cannot attain to, and we wish to tell what its nature may be. If we find that its light when dispersed by the prism shows a continuous rainbow-tinted streak, we can conclude as surely that it is an incandescent solid or fluid, as we could tell that our imagined set of keys from C to C had been swept from end to end, if we heard the whole succession of sounds, even though the instrument were out of sight. Always supposing a certain keenness of perception on the part of the auditor, it would make no difference to him whether the musical instrument were close by, or in another room, or even in another house; so long as he heard the whole succession of sounds he would know that the whole series of keys had been struck. And just as certainly the physicist can tell that light comes from an incandescent solid or liquid because the whole series of colours is present in the spectrum without break or interruption, even though the source of light be millions of miles away. As our imaginary auditor would be certain so long as he could hear the continuous succession of sounds, so