Mr. A. S. Davis on the Probable Character of Cometary Orbits. 183 



both of wave-lengths and of indices of refraction is difficult, on 

 account of the feeble intensity of the light. 



Since only the differences between the angular deviations of 

 the spectral lines are employed in the formula above given, it 

 follows that, in determining wave-lengths by the method in ques- 

 tion, it is not necessary to employ a spectrometer with a divided 

 circle and appliances for the measurement of large angles. A 

 common spectroscope will be sufficient if the observing-telescope 

 be provided with a filar micrometer, by means of which the an- 

 gular distances of any given line from two other lines of which 

 the wave-lengths are known may be measured. The researches 

 of Angstrom leave nothing to be desired as regards the wave- 

 lengths of standard lines \ and the method given may prove a 

 convenient means of determining with all requisite precision the 

 wave-lengths of metallic lines. 



XXII. Onthe Probable Character of Cometary Orbits* By A. S. 

 Davis, B.A., Mathematical Master, Leeds Grammar School*. 



THE discovery by M. Hoek that certain comets may be so 

 arranged in groups that all the members of the same group 

 have directions nearly coincident and the planes of their orbits a 

 common line of intersection, as well as the discovery by Professor 

 Kirkwood, that there exists a connexion between the aphelion 

 positions of comets and the direction of the sun's motion in space, 

 tends to confirm the opinion that the parabolic comets are not 

 permanent members of the solar system revolving in elliptic 

 orbits of great length, but are casual visitors, coming from the 

 interstellar regions of space, and, after passing through their pe- 

 rihelia, moving off, never perhaps to return again. On the other 

 hand, the fact that the orbits of most of these comets do not 

 sensibly differ from parabolas, has given rise to the opposite 

 opinion that they are really ellipses of great length. No hyper^ 

 bolic orbit of other than very small excentricity has been met 

 with, contrary to what might be expected on the supposition that 

 comets are non-periodic. For the parabolic character of a 

 comet's orbit shows that its motion at a great distance from 

 the sun is very nearly the same as that of the sun, and so is not 

 independent of the sun's motion, as it might be expected to be 

 on the hypothesis that it is not a permanent member of the solar 

 system. Supposing that the motion of comets at a very great 

 distance is independent of the motion of the sun, their average 

 velocity at a great distance relatively to the sun must be at least 

 as great as the velocity of the sun's motion in space. 



• . * Communicated by the Author. 



