ON THE EFFECT OF PRESSURE UPON THE WAVE

LENGTHS OF THE LINES OF THE HYDROGEN
SPECTRUM.1

IN

By J. WILSING.

In my paper on "The Interpretation of the Typical Spectrum of New Stars," published in the Sitzungsberichte of the Academy for May 4 of this year, I communicated the results of experiments which very clearly confirmed the effect of the pressure of the vapor on the wave-length of lines of metallic spectra discovered by Humphreys and Mohler. I there availed myself of the disruptive discharge between metallic electrodes in water, which occurred with explosive violence, and caused a rise of several hundred atmospheres in the pressure of the spark. In consequence of this there resulted in case of most lines an increase of wave-length associated with broadening, which amounted in some instances to several hundredths of a tenth-meter. The following experiments now show that also the lines of the hydrogen spectrum similarly undergo an appreciable displacement. toward the red on increase of pressure.

I have already indicated in the paper cited that the disturbing broadening and poor definition of the hydrogen lines with increasing pressure occur only when the temperature of the discharge simultaneously increases, and that it might therefore be possible to make exact determinations of the wave-lengths if at the same time provision was made for keeping down the potential by decreasing the distance between the electrodes. I was thus able by measuring spectrograms of the "second" hydrogen spectrum at atmospheric pressure to make it seem likely that appreciable displacements of certain lines do actually occur in the direction indicated, but on account of the slight difference of pressure a final conclusion could not be reached.

I

1 Sitzungsberichte der Akademie der Wissenschaften zu Berlin. Joint session on July 27, 1899.

2 This JOURNAL, 10, 113, 1899.

A very noticeable displacement toward the red of the hydrogen line with increase of pressure is proven with certainty by the following experiments, to the success of which the assistance of Dr. Eberhard has contributed much.

It was my intention, starting from the same point of view as in the experiments on the change in wave-length of metallic lines, to investigate first the position of the hydrogen lines in the arc spectrum when the carbon poles dipped in water, but the voltage of the small Siemens dynamo of early type at my disposal was not sufficient for the production of an intense hydrogen spectrum. On using the high potential spark between metallic electrodes in water, the intense continuous spectrum overpowered the weak bands of the hydrogen spectrum, while carbon electrodes, which give a less bright continuous spectrum, are rapidly deformed in water, so that a uniform illumination of the slit of the spectrograph could only have been obtained with greater sparking distances than I could employ.

When the discharge took place in air, however, as soon as the carbons were moistened with water, HB especially appeared as a broad, faint, bright band, in the middle of which a comparatively fine dark line could be seen. Liveing and Dewar' observed the flashing out of the hydrogen lines Ha and Hẞ when they put a drop of water on the electrodes, but they did not perceive any phenomena of reversal.

With the spectrograph employed for the above mentioned. metallic spectra, photographs of the spectrum of the discharge between moistened carbon electrodes and of the spectrum of a hydrogen tube were now made upon the same plate. The displacement of the absorption line Hẞ toward the less refrangible end of the spectrum was clearly visible on these plates. The following summary gives in the first column the amounts in revolutions of the micrometer of the displacements measured. upon five plates, in the second column their values in wavelengths, together with weights and remarks:

"Note on the reversal of hydrogen lines; and on the outburst of hydrogen lines when water is dropped into the arc." Proc. R. S., 35, 1883.

The width of the bright band HB, diffusely bounded on both sides, amounted to more than 5μμ, and its position could not be measured with sufficient accuracy on account of its diffuse edges. The mean width of the absorption line was 0.8μp. On one plate it was more sharply bounded toward the violet, on the others it was equally diffuse on the two sides. The different photographs necessarily show differences in respect to superposition, sharpness and displacement of the dark lines, since the image of HB probably results from the superposition of the radiation of many layers in which pressure and temperature differ considerably, and since further the phenomenon of reversal appears in different degrees of distinctness corresponding to the varying conditions of the discharge. Taking this into consideration we may regard +0.11μμ as the mean amount of the displacement of the HB line in the spark spectrum from the corresponding line in the spectrum of the tube under the given conditions.

Of the other hydrogen lines, Ha was easily visible in the spark spectrum, Hy appeared to be present on some plates as a faint and excessively broadened band, but the strongly developed. carbon lines in the neighborhood were disturbing. The bands corresponding to Hồ and He, which were presumably even more broadened and diffuse than the less refrangible hydrogen lines, were completely concealed by the strong calcium lines at λ423μμ, 397μμ, 393μμ, and the cyanogen band, whose less refrangible edge has the wave-length 422μμ.

KGL. ASTROPHYSIKALISCHES OBSERVATORIUM,

Potsdam.

ON THE PRESENCE OF OXYGEN IN THE ATMOS-
PHERES OF CERTAIN FIXED STARS.1

By DAVID GILL.

IN a paper read before the Society on April 8, 1897, and in a subsequent paper, Mr. Frank McClean draws attention to the grouping of lines other than those of helium and hydrogen in the spectra of 8 Scorpii, B Canis Majoris, B Centauri and B Crucis, suggesting that the close correspondence between the grouping of these extra lines and the known lines of oxygen, points to the probable presence of that gas in the atmosphere of these stars.

In the latter paper he writes: "The most remarkable correspondence is in the case of the large group on either side of Hồ. A slight shift of about a tenth-meter is required to bring the groups into identical positions. However, the close similarity of the whole grouping of the two spectra as they appear on the plate, admits of little doubt that the extra lines actually constitute the spectrum of oxygen. If this be established, the spectrum of the first division of helium stars would be due to hydrogen, helium, and oxygen.”

In his subsequent work3 Mr. McClean concludes: "Taking everything into account, the succession of coincidences between the extra lines of Crucis and the oxygen spectrum can only be accounted for on the basis of the extra lines being in the main actually due to oxygen."

This conclusion does not as yet appear to have been fully accepted by spectroscopists, partly because from the low dispersion used the lines of the groups are not separately shown. It is very generally known that the instrumental equipment of the Royal Observatory at the Cape has recently Proc. Roy. Soc., 55, 196, 1899.

Proc. Roy. Soc., 42, 418, No. 386.

3 Spectra of Southern Stars. (Stanford, London, 1898.)

!

been enriched by a complete equipment for astrophysical research, the whole being the munificent gift of Mr. McClean, F. R. S.

The slit-spectroscope, for attachment to the photographic refractor, reached the Cape in the middle of January last, and I resolved that its first published work should deal with Mr. McClean's interesting discovery.

As a complete account of the instrument and its Observatory will be subsequently published, it may be sufficient for the present to state that the object-glass of the photographic telescope has an aperture of 24 inches, and focal length of 22 feet 6 inches, its minimum focus being, at present, for rays about midway, between Hẞ and Hy.

The collimator of the spectroscope has an aperture of 24 inches, and focal length of 221⁄2 inches, so that a cylinder of parallel rays 2 inches in diameter falls on the prisms, and the latter are of sufficient size to pass the whole of the rays which form the image of the spectrum on the sensitive plate. The instrument is provided with two camera-telescopes of 24 inches aperture, one being of about 36 inches focal length, the other of 16 inches.

Only the larger of the two camera-telescopes has been employed in the after-mentioned observations.

There are two cast-iron prism boxes; one of them contains three prisms of about 60° each, which for rays near Hy produce a deviation of 180°, so that the camera-telescope becomes parallel in the reverse direction to the slit-telescope. The other prism box contains a single prism of 62°. The prisms in both boxes are fixed, without screw adjustment, in minimum deviation for Hy. The collimator is in the axis of a solid drawn steel cylinder-the latter attaching by a flange at one end to the butt end of the telescope; a cast-iron plate attaching to the flange on the other end of the cylinder carries either one or other of the two prism boxes.

The slit-slide and the 60° prism for reflecting the comparison. spark on the slit (made on the plan of the Lick spectroscope)