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before long he would 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 him 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, the present writer had long before employed this method of illustrating the subject. But it was a new thing—to him, 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 he found the audience grasping at once the points he 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 the writer 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 referred to, and have perhaps often seen its action scientifically explained, but have yet no clear and definite ideas of the nature of the evidence it sup
plies, or of the reasons on which men of science base their acceptance of such evidence.
Everyone 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 rainbow-tinted 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 rainbowtinted streak, and that only the light from such bodies will exhibit a continuous streak of light from deepest red to deepest violet. This is an experimental fact. Now suppose there is some self-luminous 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 the physicist, using
the spectroscope, is certain as long as he can see the continuous spectrum.
Let us now consider another case. Suppose certain notes only of those forming our continuous gamut of sound were struck in quick succession. An auditor would be able to tell what those notes were. He would recognise them as a definite set of notes. If the same series were struck simultaneously, either by the fingers of a musician or by some instrument constructed for the purpose, the auditor would be able, if he were a practised musician, to tell the exact set of notes thus sounding simultaneously. But it will be convenient for the purposes of illustration to consider the case of a succession of definite notes;? because everyone can
Recently, attention was directed, in the Quarterly Journal of Science, to the analogy between sound and light. It appears to us that although such an analogy undoubtedly exists, an attempt was made to push the analogy farther than the evidence warrants. In the spectrum we have a succession of colours precisely as in the gamut we have a succession of notes; but the succession in one case depends on position, in the other on time. The colours of the spectrum are seen to succeed each other as the eye passes from the red end to the violet end; the notes of the gamut succeed each other as they fall one after another on the ear. Hence a chord in music corresponds to a spectrum compounded of several prismatic lines. So far the analogy may be followed. But we cannot reasonably extend the analogy so far as to assert that there is anything in the theory of colours corresponding to the effects produced by concordant or discordant sounds. If three successive notes of the gamut are sounded together we have an unpleasing sound ; but if red, orange, and yellow lights are commingled, the resulting light is not unpleasing
-no eye can, in fact, distinguish it from pure orange light. And similarly of other combinations. Three or more colours corresponding (so far as the waves of light are concerned) to a pleasing musical chord, produce together a colour which is not a whit more pleasant than the colour produced by mixing three or more colours corresponding to a discordant combination of sounds. Who would pretend to say, for
understand how such a succession could be recognised, whereas the musician's power to recognise the component notes of a chord is less common.
Now it is found that when the light of a glowing gas is examined with the spectroscope, it is resolved into a definite number of coloured bands or lines. Some gases show only one or two lines; others many; others, again, show broadish bands, with dark spaces between them. But we may assert in a general way of the spectra of glowing gases that they are discontinuous under ordinary circumstances. Further, setting aside as before certain exceptions, the consideration of which belongs to a more advanced stage of the science, we may say that each gas has its own family of lines, which always make their appearance when the light of
instance, that the coloured rings seen when a lens of glass is pressed against a glass plane, or the colours seen in a bubble, are less pleasing to the eye than the colours of the prismatic spectrum ; yet the latter are pure, while the analogue of the former colours in sound would be a series of noises as painful to the ear as saw-filing.
It may be questioned, indeed, whether there is such a thing in nature as an ugly colour, that is a colour which, apart from some association of ideas, is painful to the healthy eye; whereas only certain combinations of sound are pleasing to the tutored ear; and many are essentially painful even to those who have no musical taste.
On the other hand, it is worthy of notice that whereas certain combinations of pure colour by juxtaposition are essentially unpleasant, it may be questioned whether any sequence of simple notes can be so regarded. It would, however, require more space than is here at our disposal to discuss this question, since both the parts into which it divides itself are associated with questions of some difficulty. It would be no easy task to determine either on the one hand the essential principles on which the pleasing or unpleasing effects of juxtaposed colours depend (the laws of complementary colours being by no means sufficient for the purpose), or on the other, the principles which render certain sequences of sound more or less pleasing to us.