be so well seen, since the background, as I have said, will be very much brighter than in those cases. They can be made as conspicuous only by an increase of dispersive power; hence the importance of constructing prismatic batteries of great dispersive power.

In connection with this portion of my subject it is necessary to remark that the bright lines seen in the prominence-spectrum are not uniformly wide throughout, but commonly are wider close to the Sun's limb. This circumstance will be referred to more at length further on; but it is proper to state in this place, that this increase of width is held to indicate an increase of pressure, because the researches of Plucker, Hittorf, Huggins, and Frankland have shown that the spectral lines of, hydrogen grow wider as the pressure at which the gas exists is increased.

And now, lastly, it remains that I should explain what has been justly regarded as the most wonderful of all applications of the powers of spectroscopic analysis -the measurement of the velocity of recess or approach of stars, or other self-luminous objects moving with very great rapidity.

Reverting to fig. 21, the reader will see that the violet rays are most affected by their passage through the prism, the red rays least. Now, it has been demonstrated by the careful mathematical analysis of the

*

* I refer here to the investigations of Cauchy, and Baden Powell, and others (see specially Cauchy's Mémoire sur la Dispersion), not, of course, to the proof that when the differences of velocity are admitted, differences of refrangibility are accounted for. The latter may be regarded in fact as self-evident.

motions of light-waves that this difference of refrangibility is due to the different velocities with which the longer light-waves forming red light, and the shorter light-waves forming violet light, travel (respectively) through material media. The shorter waves travel more slowly than the long ones, and the difference is the greater according to the density (or approach to opacity) of the medium. So that, in fine, the part of the spectrum formed by light of any order depends on the wave-length of that light; and if under any circumstances the wave-length could be altered, then the light of that order would no longer occupy the same portion of the spectrum, but would pass nearer to the violet end if the waves were shortened, and nearer to the red end if they were lengthened.

Now, so far as we know, it never happens that lightwaves of a certain length are really modified. Precisely as waves of a certain breadth propagated along a canal are not found to change their breadth as they proceed, or, again, precisely as a sound of a certain tone does not change in tone as it travels onwards, so lightwaves of a certain length or order do not as they travel through ether, or through material media, become changed into light-waves of some other order.†

In the ether of space they travel of course with appreciably equal velocities; otherwise the satellites of Jupiter, after emerging from eclipse, would show the same changes of colour that we see in a metal heated from a red to a white heat.

I have sometimes been inclined to suspect, however, that under certain circumstances of excessive agitation within the substance of the source of light, the wave-length might be altered, precisely as waves travelling along a canal might be modified in length by the action of

But there is a circumstance which may cause the light-waves to appear to change in length. Supposing the source of light is approaching or receding at a very rapid rate at a rate which bears an appreciable proportion to that of light-then the length of the light-waves must needs appear modified-shortened when the source. of light is approaching, lengthened when it is receding. The same will also hold if the observer be carried very rapidly towards or from the source of light. To see that this is so, it is only necessary to consider that more light-waves must necessarily reach the observer in a given time when the source of light is approaching, than when it is at rest (with respect to him), and fewer when it is receding. They must then in one case succeed each other more rapidly, and so seem to be separated by shorter intervals, while in the other they must succeed each other more slowly, and so seem to be separated by longer intervals.*

the cause which gave them birth. When we know that the c line of the prominences has been observed to be tranquil, while the F line has been broken, the idea is certainly suggested that those molecular motions within the substance of the hydrogen of the prominences, which produce that part of the light corresponding to the F line, may by some violent action be so far modified that the observed disturbance of the wave-length corresponding to that particular line may be brought about. It seems difficult to understand how, under any other circumstances, one line of the hydrogen should be undistorted, while the other is, to use Professor Young's description, 'absolutely shattered.'

The principle on which this brief but sufficient explanation depends admits of several illustrations. I do not know of any which more clearly exhibits the true nature of the principle than one which I employed in the first matter I ever wrote for publication, a paper on 'The Colours of the Double Stars,' which appeared in the Cornhill for December, 1863. I quote the portion referred to:-' Let the reader imagine

Now, Doppler, who first called attention to this circumstance, supposed that an alteration of colour would

himself on the bank of a canal, observing a series of waves uniformly propagated along the stream. . . . A very simple method will suffice to determine the breadth of the waves with any required degree of accuracy. Let the observer, fixing his eye on a certain wave, walk any measured distance (say 100 yards) at the same rate as the wave is moving. Suppose he accomplishes this distance in 65 seconds. He knows then that the velocity of transmission of the waves is 100 yards in 65 seconds. Let him now, standing still for 65 seconds, count the number of crests that pass him in that time. Suppose 360 pass him. Then, from his first observation, he knows that the first which passed him has travelled 100 yards from him. Within that distance all the 360 waves are uniformly distributed. Thus the breadth of each is 1-360th part of 100 yards, or ten inches. This result is perfectly reliable if, during his second observation, his position on the bank has been unchanged. But let us imagine that he has made this observation from a truck-on rails by the canal's edge-and that, unnoticed by him, the truck has glided uniformly along the rails. First, suppose that this motion has taken place in a direction contrary to that of the waves, and that while he is counting the passing crests the truck glides a distance of 20 yards. It is evident that when the last wave passes him, the first is 120 yards, instead of 100, from him. Thus the 360 waves are distributed over 120 yards, and the true breadth of each is 1-360th part of 120 yards, or 12 inches. If, on the other hand, the truck had moved over 20 yards in the same direction as the waves, it is equally obvious that the 360 waves would be distributed over only 80 yards, and the breadth of each would be only 8 inches. Similarly, at whatever rate the truck moves, it is evident that the observer can no longer depend on the result of his observations. If it moves in a direction opposite to that in which the waves travel, they appear narrower; if it travel with them they appear broader than they really are. Indeed, it is not difficult to conceive the truck to move in the same direction and at the same rate as the waves travel, in which case (if we could suppose the observer to remain unconscious of that motion) all undulation would appear to him to have ceased, and the water to have a waved but unmoving surface.'

This account illustrates in a very direct (and I think effective) manner the effect of the approach or recess of a source of light. We see that the effect depends on the ratio which the velocity of approach or recess bears to the velocity with which the waves travel. It will be seen at once that equal velocities of approach and recess produce equal but not

result; and this would, in fact, be the case (though the alteration would under any conceivable conditions, be wholly inappreciable) if the source of light emitted rays of a certain order only. But in the case of such a source of light as a star-or the Sun-no change of colour could be produced, because though—to take the case of approach-the red light would be shifted towards the orange, while a portion of the violet would disappear, yet heat rays from beyond the red end would become visible as red rays through being shortened, and so the spectrum would be complete as before. A similar result would follow in the case of recession.

But the presence of dark lines in a spectrum gives the observer a far more effective means than mere change of colour would supply of determining the approach or recession of a source of light. If some

corresponding effects. For, by the supposed velocity of 20 yards in 65 seconds, the length of the waves is increased or diminished by two inches, but the increase is only in the ratio of 6 to 5, while the decrease is in the greater ratio of 5 to 4. Corresponding to this, we have the fact that the approach of a star at a given rate produces a greater relative effect on wave-lengths of any order than the recess of the star at the same rate would produce.

Of course, in the interesting case of stellar approach or recess, the problem is somewhat complicated by the Earth's own motion, which does not take place in the direction of the star, nor necessarily in the same plane. The successful measurement of the velocity with which Sirius is receding from us is a problem of such interest that I may be permitted to note a very small contribution of mine to the work, in the determination of the formula for eliminating the effects of the Earth's motion (v) viz.-Earth's motion towards starv cos A sin (1-1) where and are the respective longitudes of the star and the Earth, and A is the star's latitude. The contribution has no particular value, but, such as it is, it chanced that I made it.