clues, a new study came into being, that of solar physics. To-day we know more of the constitution, the temperature, and physical characteristics of a body ninety millions of miles away, and for ever inaccessible to man, than our forefathers three or four generations ago knew of the earth itself. Not merely this, but the spectroscope has reached out to tell the same story of the stars; to-day the stellar bodies are divided according to their apparent temperature into different types-solar or yellow stars like our sun; white or bluish stars like the great Dog-star, Vega, and others; red suns like the bright star of the constellation of Hercules; the subdivision is often carried much further. From out of solar physics has grown a yet broader study, that of stellar, or, if one prefers, of cosmic physics. Amid the multiplied interests of man there are surely none more distantly removed from his primitive pre-occupations for food and shelter than this. But the revelations of the spectroscope were not to stop with chemical and physical conditions. By an extraordinary circumstance, grounded in the very nature of light itself, it was to do more; it was to prove an independent method of determining solar motion. There is much in the economy of science that resembles that of our workaday and business life. Very often it has happened that a loan of fact or theory from one branch of scientific investigation has later been returned with heavy interest. This was true in the present instance. It was the motion of the earth which first revealed the finite velocity of light. Later, as we have seen, this was turned about in explanation of the annual aberration of the stars. The return went further. If light be a form of wave motion, it follows that if the body which sets up these light waves be itself in motion, in sufficient velocity, this would occasion some disturbance in the behaviour of the light it transmits. You find a familiar analogy in sound. If while an express train is travelling at a high rate of speed the engineer blows his whistle continuously, this will seem to cause a change of pitch in the sound, accordingly as the train is travelling towards or away from a bystander. The fact is one of everyday experience. The case with light is a little different. Not only does it travel at nearly a thousand times the velocity of sound, but its vibrations are crosswise to the line in which it travels. The motion of the source of light does not occasion a change of pitch-that is to say, of the light colour-but this movement is disclosed by a slight displacement in the position of the lines of the spectrum. The placing of the lines is determined by the wave-lengths of the different rays. If the source of light be approaching, the waves will be crowded a little closer together, with the result that the lines will be pushed up a little farther towards the violet end of the spectrum. Conversely, if the source of light is flying away from us, with sufficient speed, the wave-lengths will be drawn out a little, the spectral lines will be displaced slightly towards the red end. By a marvellous refinement of measurement, this displacement can be utilised to reckon the speeds of stars moving directly in the line of sight-that is to say, which have a motion radial to that of the earth and the sun. This is accomplished by means of photography. If the spectrum of a moving star be registered upon a photographic plate, side by side with that of some source of light within the laboratory, whose lines exactly correspond with those of the star, the minute change of position due to the star's motion may be observed. By this means it has been possible, to some extent, to check the calculations as to the speed of the stars reckoned from the measures of parallax and of proper motion across the line of sight, or at least to render these calculations probable. If, for example, the speed of a number of stars of about equal magnitude, and therefore probably of about the same distance from the earth, be computed first by the earlier method, then as many more by the spectral method, we may compare the averages. If they roughly agree, it is evident that in neither the one instance nor the other are we dealing with optical illusions. The chances against this concurrence being the result of a mere hazard has enormously increased as the number of motions thus reckoned increases, and with this increase the concurrence grows closer. Were it possible only to reckon the speed of the stars by means of their parallax, it might still be open to supposition that these computations were grounded upon some periodic disturbance like that of stellar aberration and nutation. Were the estimates grounded only upon the displacement of the lines of the spectrum, the difficulties which the latter involves, the extraordinary delicacy of observation required, might equally leave the results open to doubt. Their substantial concurrence-that is, that the average speed of a hundred stars reckoned by the one method and a hundred more by the other, is not vastly different-brings a high degree of assurance. Some day or other the concurrence of these two methods may result in a reckoning yet more sublime—that is, the orbits of the stars. It is obvious at a glance that the spectrograph registers only the part of stellar motion that lies directly in the line of sight; while, on the other hand, of proper motions we can know nothing, save that part of the motion which lies at right angles to the line of sight. If stars could be found having both a large radial motion and a large proper motion, likewise a measurable parallax, it would then be possible to estimate their actual speed, and to plot their paths as well. Obviously, however, such a condition could not obtain. If the apparent motion were large, the spectral displacement would be small, and vice versa, so the one or the other would be too minute for registry, unless perchance the speed were prodigious and the placing of the star just right for such observation. This is a problem of the future. It is clear that if a long series of observations could show, for example, that either the apparent or radial motion was growing greater or growing less, here at least would be a slender basis from which some sort of inference might be made. In following the disclosures of the spectroscope, especially in its amazing invasion of the invisible, we shall see that some stellar orbits have been found, not of the sort we have been considering, but quite another. Meanwhile, we are now in a position to sum up man's long endeavours to orient himself and his world amid the cloud of stars. The effort is considerable. The mind grasps with ineffectual fingers a vivid realisation of the teachings of the new time. Distances, figures, lose all meaning in their vastness. They leave only a blur. Let us try a little to sharpen the image. |