Spectroscopic investigation has shown that this comet contains luminous carbon, or carbon compounds its spectrum, together with that of Brorsen's comet, is seen in Fig. 87. This gives that modification of the carbon spectrum which we obtain when the electric spark is passed through olefiant gas, the coincidence of the bright lines of Winnecke's comet with those of the spark taken in olefiant gas being clearly seen. In order to obtain an exact comparison of the lines of the comet with those of incandescent olefiant gas, the arrangement shown in Fig. 89 was employed. This consists of a glass bottle, a, converted into a gasholder containing the olefiant gas. This was connected with the glass tube, b, through which the gas passed, and into which two platinum wires, e and f, had been soldered. This tube being then placed before the mirror of the spectroscope c, the light of the spark, passing through the gas in the tube by means of the wires, was reflected into the instrument d, and its spectrum was seen immediately below that of the comet. The two sets of bands were not only found to agree precisely in position, but they corresponded in their general characters, and in their relative brightness. Hence we can scarcely doubt that carbon is really the cause of these bright lines; but whether the carbon is present in the free state in the comet, or in the state of combination, cannot be as yet definitively decided: nor can we explain how carbon can be transformed into the gaseous state, or heated so as to become luminous, unless indeed it be present in the form of a hydrocarbon which becomes ignited or enters into combination with some other constituent of the comet by the action of the sun's heat. That carbon is an element widely distributed throughout the universe we learn from the fact that it has been detected in considerable quantities in the extra-terrestrial matter of meteoric stones. This observation gains a further interest, as it appears probable that the orbits of many comets are identical with the paths of the recurring meteors. Hence an intimate connection doubtless exists between the comets and falling stars, so that meteorites may perhaps consist of condensed cometary matter. These observations have received further confirmation from the fact that two comets appearing in 1871, one that of Encke and another discovered by Winnecke, have each Leen shown by Huggins to give a spectrum of three bright bands again corresponding to the carbon lines. I have still to speak of another result of these interesting experiments of Dr. Huggins. Not only are we in a position thus to determine the constitution of the stars and of the nebulæ, but, strange as it may appear, we can actually, by these observations, get some ideas respecting the relative motions of these bodies and our earth. It is impossible in the time at my disposal to explain to you the mode in which philosophers or physicists have arrived at the conclusion, originally propounded by Doppler in 1841, that, when a luminous body is approaching another very rapidly, the kind of light which is received on the retina from that body moving at a very great speed differs in some respects from the light which the retina would receive were that body at rest. An illustration from sound may perhaps render this matter more plain. If in a railway train you listen to the whistle of the engine of another train which is meeting you, you will notice that as the two trains approach the pitch of the note of the whistle alters.1 This is because (owing to the sound being produced by the vibration of the particles of the air), when the two trains are approaching each other, the waves of sound are, as it were, forced together and fall more rapidly upon the ear than they would do if the two trains were in a state of rest. The same thing happens with regard to light. If the one object which is luminous is approaching the retina very rapidly, the vibrations causing light will fall more frequently on the retina than if the bodies were at rest; and then the position of the dark lines will be shifted in the direction of the most refrangible rays; whilst, if the bodies were separating, the shifting would take place in the direction of the red or least refrangible rays. Dr. Huggins has actually found that in some of these stars there is a slight disturbance in the position of the hydrogen line F: he first proved that it is really hydrogen which is present, and then he showed that there is a slight deviation observed between the hydrogen line and the line existing in the star; and hence he comes to the conclusion that there is motion of recession between the earth and what we call a "fixed star." Here you see 1 An exact experiment of this kind was made in 1845 by Ballot of Utrecht, in which the alteration of tone for a given velocity was determined. a diagram (Fig. 90) showing the slight deviation which the line F exhibits in Sirius' light. You see that the narrow line of hydrogen from the vacuum-tube does not coincide with the middle of the Sirius F line, but crosses it at a distance from the middle, which may be represented by saying that the want of coincidence is apparently equal to about one-third or one-fourth of the interval separating the two D lines. Hence the F line has been distinctly deviated towards the red rays, and the vibrations proceeding from the star must have been retarded in their passage; or, in other words, there is a Hydrogen at pressure Solar Spectrum Line F Spectrum of Hydrogen in Vacuum tube FIG. 90. ; motion of recession between our earth and Sirius of such a nature that the wave-length of the F ray has been increased by the 0.109 millionth part of a millimetre. The velocity of this recession can easily be calculated. Light travels at the rate of 185,000 miles per second the wave-length of the F line is 486.5 millionths of a millimetre now the velocity with which the two bodies move away from each other stands to the velocity of light in the same proportion as the observed difference of wave-length does to the wave-length of the particular ray; or |