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In both experiments the altered light is of lower refrangibility than the incident light-another instance of degradation of energy.

The point I shall first take up in next lecture is the point left unexplained to-day,-how it is possible for a line which was originally dark in the solar spectrum to broaden out and become bright, and then for one portion to become dark while the other portions remain bright.

LECTURE X.

SPECTRUM ANALYSIS.

Change of colour of Light by relative velocity of source and observer. Analogy from Sound. Causes of broadening of spectral lines. Spectrum of Solar Corona; of Double Stars; of Comets. Probable nature of Comets; of Saturn's rings; of the Zodiacal Light.

You remember I closed my last lecture by pointing out to you a diagram of a portion of the solar spectrum, in which we had side by side a bright line and a dark one, due to the same substances, namely, hydrogen. I told you that there is a very beautiful point of theory involved in the explanation of this phenomenon, and I proceed to give it.

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Let us take the simplest possible analogy. Suppose, for instance, that we had Mr. Perkins' steam-gun, and suppose it projecting bullets in the same direction, succeeding one another once every half-second. Then, if a target were held in the path of these bullets, it would of course be struck 120 times minute. But suppose that the target were to move up towards the gun, while the gun still kept on discharging the bullets at the same rate, it is obvious that it would meet more bullets in the course of a minute than it would meet if it were standing still. If you were to withdraw the target gradually, keeping it always however in the line of fire, you would get fewer bullets per minute; and if you

were to make it move away from the gun at exactly the rate at which the bullets are coming, then no bullets would reach it at all. One bullet would be in its neighbourhood, and would remain constantly at the same distance from it; for, in fact, the target and the bullet would be moving with the same rapidity.

Precisely the same thing may be observed in passing over a set of waves. If you are steaming through a set of waves in the direction in which the waves are going, it is quite conceivable that you may be steaming so fast as to be riding on the crest of a definite wave all the way; but steam a little more slowly, and you will see waves gradually passing you; steam still more slowly and a greater number of them will pass you per minute. If, on the other hand, you are steaming so as to meet the waves, then you meet more than if you were not moving. The faster you go you meet the more waves per minute; and there is absolutely no limit to the number you may meet per minute, if you could only move fast enough to meet them. Now the impression, be it of pitch or of colour, that is produced upon the ear by sound, or upon the eye by a luminous radiation, depends entirely, so far as our present purpose is concerned, upon the number of these waves which meet them per second. Therefore, if we are moving towards a sounding body which is giving out a particular note, the number of waves which reach our ear per second will be greater than it would be if we were standing still, or (generally) if we were at rest relatively to the body. And as a higher note corresponds to a greater number of waves reaching our ear per second, it is obvious that in the former case, whether we are moving to the sounding body or the sounding

body is moving to us, there will be a greater number of waves reaching our ears than if we were at relative rest; so that we should perceive a higher pitched sound than what is actually given off by the sounding body. The experiment has been made by the help of a railway engine-first in Holland, and since in other countries by stationing upon the engine a trumpeter, who had beside him a musician to control exactly the note that he should play. The musician, of course, was moving along with the trumpeter, and therefore heard precisely the note that the trumpet was sounding. The sound, however, was also heard by other musicians who were placed at the side of the line, and they noted that the faster the engine came up to them, the higher did they hear the note which was played by the trumpet; and the faster the engine went away after passing them,-the faster it retreated from them,-the lower did this note appear to be. I have no doubt that you at all events those of you who have paid any attention to musical sounds-will be able at once to perceive this effect by means of such a simple instrument as this tuning-fork, even with such comparatively slight velocity as I can give it by swinging it in my hand. For the success of an experiment of this kind, it is better that you should close your eyes, in order that you may not associate the result with any movement which you may observe on my part; and I shall endeavour to perform the experiment without making any noise which might indicate to you how I am moving, or whether I am moving, the apparatus at the instant. [Experiment shown.] You notice, then, that during the interval that I allowed the fork to sound, there was a period at which its pitch appeared to rise;

then immediately afterwards it appeared to fall; then it rose again, and so on. We had a musical sound which was alternately higher and lower in pitch as I sharply moved the vibrating fork to or from you, and then, when the fork was held steady, we had the original sound. Now, precisely the same thing happens with regard to waves of light. If you move so as to meet more waves of light in a second, that will correspond to an impression upon your retina of a higher order of colour than if you were not moving to meet those waves, or if the body which was sending those waves to you were not moving towards you. Thus you see that the light which comes to us from a star is capable, not only, as I pointed out in my last lecture, of showing what chemical substances are incandescent in the atmosphere of the star, whether as giving out light on their own account or as absorbing portions from an otherwise continuous spectrum, but is also capable of pointing out to us whether the star is moving to us or from us; or still more minutely, whether a portion of its atmosphere is moving on the whole from us, and another portion on the whole to us. The first application of this by the spectroscope to the study of the relative motion of a star with reference to the solar system, was made by Mr. Huggins with reference to the dog-star. Of course, in order to find out from such experiments (which tell us only the relative velocity of the earth and the star) what the actual velocity of Sirius is with regard to the sun, it is necessary to consider in what part of its orbit the earth is during the observation, because when the earth lies in a line from the sun, making a right angle with the line drawn to Sirius, the earth is moving much faster or much slower towards Sirius than the sun is moving. On the other