The value of I at Meanook is the mean of all the observations made during the month without regard to the time of day.

The value of H at Meanook is the mean of two observations made about the middle of the month, and the Z is obtained by the formula Z H tan I.

W. E. W. J.

ASTRONOMICAL NOTES

THE ABSORPTION SPECTRUM OF THE NOVA.-One of the most remarkable features of the spectrum of novæ or temporary stars is the presence at a certain stage in their development of absorption lines displaced greatly toward the violet end of the spectrum. In the case of the elements hydrogen and helium they appear as strong lines forming sharp boundaries to the broad emission bands which are the principal characteristic of the spectrum of such stars. Occasionally they are double and are subject to great variations in character and intensity. In addition to the lines of hydrogen and helium a large number of absorption lines has been observed in the spectra of the three brightest novæ of recent years, Nova Persei of 1901, Nova Geminorum of 1912, and Nova Aquilæe of 1918. The identification of many of these lines and some features of their displacements form the object of this communication.

Nova Aquila was first observed at Mount Wilson on June 8. At this time the spectrum was essentially continuous, with a few broad and hazy absorption bands superposed upon it. Most of these were due to hydrogen and helium and were displaced over 20 angstroms toward the violet. Two nights later the entire spectrum had changed: a large number of comparatively narrow absorption lines had appeared and the hydrogen and helium lines had become double. This spectrum had so many points of similarity to that of certain stars that a comparison was instituted with a photograph of a Cygni, a star of early type with exceptionally strong enhanced lines. It then appeared that a considable number of lines could be identified as common to both spec

tra provided a displacement of about 23 angstroms at X4500 were assumed in the case of Nova Aquilæ. A large proportion of these lines are enhanced but some of the stronger arc lines are also represented. In the region of spectrum between X3900 and Hẞ ninety lines have been identified with considerable certainty, and the origin of many others may be assigned with only slightly less probability.

We find, accordingly, the surprising result that the displacements of the lines in all of these stars are directly proportional to wave-length and divide themselves into two pairs of equal amount. Of these the first pair of stars has exactly twice the displacement of the second, and it is perhaps a significant fact that Nova Aquile and Nova Persei were much brighter stars apparently, and probably intrinsically as well, than Nova Geminorum and Nova Aurigæ.

A very peculiar phenomenon in the case of Nova Aquila is the progressive increase in the values of the displacements of the absorption lines at successive dates.

In the case of Nova Geminorum, therefore, the displacement of this component of the hydrogen lines is almost exactly twice. that of the less refrangible component and the numerous metallic lines, while in Nova Aquile it is one and one-half times as great. In other words 11, 22 and 33 angstroms, in the harmonic ratio 1, 2 and 3, represent very closely all the displacements found among the absorption lines in the spectra of these four stars with the exception of the four narrow nearly undisplaced lines of calcium and sodium from which the radial velocities of the stars may be derived. These lines, like the broad emission bands of hydrogen and helium, may perhaps be considered as belonging to the stars themselves, while the greatly displaced absorption lines originate in an outer envelope possibly detached from the body of the stars.

It is certain that no adequate explanation has been offered as yet to account for these immense displacements. Were the hydrogen and helium lines alone involved it is possible that some of the complex phenomena of self-reversal under conditions of marked variations in density and pressure might, at least in part,

be responsible. The presence, however, of a great number of comparatively narrow single absorption lines unaccompanied by emission bands cannot be explained in this way. Their character, as well as the law of variation of displacement with wavelength, also precludes the agency of pressure (assuming that negative displacements can be produced by pressure) and the Zeeman effect. No dependence of anomalous dispersion upon wave-length is known, even if it were adequate to produce lines of such a character with such displacements.

In some respects the Doppler effect accounts most nearly for the results observed. Motion in the line of sight would produce displacements directly proportional to wave-length and might leave the spectral lines well-defined. The velocities involved, -750 km. in the case of Nova Geminorum and Nova Aurigæ, and 1500 km. for Nova Aquila and Nova Persei, though large, are of the order found for some spiral nebulæ, and not many times greater than those observed in some of the solar promin ences. If the more refrangible component of the hydrogen, helium and calcium lines is considered, the velocities amount to -1500 and -2200 km. respectively. These values are for motion in the line of sight. If a generalized form of the Doppler principle is used a part or the whole of the effect might be referred to a rapid change in the thickness of stratum of gas producing the absorption, or in its refractive index. This hypothesis was suggested by W. Michelson to account for the high velocities observed in solar prominences, and by Paddock in the case of Nova Aquila. Some objections to this view are the rate of change in refractive index or in thickness of the gas which would be required, especially since a decrease of index would be needed to produce displacements toward shorter wave-lengths; the harmonic relationship found among the displacements; and the relatively narrow character of the absorption lines.

The last two objections might also apply to motion in the line of sight. The suggestion, however, may be made that the absorption lines are produced in a shell of gas which is moving radially outward from the star with a high velocity. If the size

of this shell is large as compared with that of the body of the star it is evident that an area of the shell only equal to that of the star would be seen in projection against the latter and would give absorption lines, and that all of the gas within this area would have large components of velocity toward the observer. This would result in comparatively narrow absorption lines. The remainder of the shell of gas would give an emission spectrum, and the combination of the widely different velocities would result in very broad bright bands with their centres nearly undisplaced. This is in accordance with observations. With these high velocities the interval of two days between the outburst of the star and the appearance of the prominent absorption spectrum would be sufficient for the gas to reach a great distance from the surface of the star. The hypothesis would, however, leave unexplained the apparent acceleration of motion during the period of observation of the absorption spectrum; and the nearly constant character of the emission bands after the disappearance of the absorption spectrum would point rather to their origin in the star itself.

The possibility of some form of dissociation is suggested by the harmonic relationship between the displacements of the components of the hydrogen lines, and the early appearance of the nebular lines in the spectra of these stars adds interest to consideration of this nature.-W. S. ADAMS, Proceedings National Academy of Sciences, U.S. A., December, 1918.

THE following Press Despatch has just been received :--Cambridge, Mass., February 3.- Professor Edward Charles Pickering, director of the Harvard Observatory, and one of the most widely known astronomers in the country, died late to-day after a brief illness.

FALLING SNOW COLORED BY DUST.-A study has been made by Messrs. A. N. Winchell and E. R. Miller, of the University of Wisconsin, of a shower of dust which discolored falling snow at Madison and elsewhere on March 9. 1918. According