In this way the wave-lengths of the lines in the spectrum of B Crucis given in the following table (p. 280), have been determined:

Column (1) gives the wave-lengths of all known oxygen lines of intensity 3 or brighter, between 4303 and 4575, according to Neovius as well as Trowbridge and Hutchins.'

Column (2) gives the wave-lengths of all helium lines according to Runge and Paschen, contained within limits of the spectrum under observation.

Column (3) gives the results of my measurements of the negative of the spectrum of 8 Crucis, those under head I being my first essays in the measurement of any photographed spectrum, those under head II being the results of my second measurement, including all the lines which could be detected. under most careful and repeated scrutiny. Each result in series I depends on two pointings, each in series II on four pointings. The lines whose wave-lengths are given to two decimal places of the tenth-meter were measured with a magnifying power of fifteen diameters, those given to one decimal place with a power of only three diameters, the lines of the latter class being very faint and only certainly visible under a very low power. When possible, different iron lines were used in series I and II for determination of the wave-length of the stellar lines.

The observations were not arranged for determination of motion in the line of sight, but the exact coincidence of the star line 4417.06 with the air (oxygen) line, and the general agreement of the stellar hydrogen and helium lines with their known wave-lengths, tend to show that the relative motion of B Crucis. to the Earth on February 21, did not exceed ± 3 kilometers per second. On that date the Earth in its motion round the Sun was moving towards ẞ Crucis with a velocity of 18 kilometers per second, and consequently B Crucis is probably receding from the Sun with a velocity of 18 km ± 3 km per second.

The whole of the known helium lines within the measured IWATTS' Index of Spectra, Appendix E.

2 This JOURNAL, 3, 10.

range of spectrum are unquestionably present, as also are all known oxygen lines stronger than intensity 4.

The exceedingly faint lines:

ß Crucis 4253.9 may be coincident with Neovius 4254.1: 4253.42, and

=

T. and H.

ẞ Crucis 4303.0 or 4304.0 may be coincident with Neovius 4304.4 = T. and H. 4303.8

but these coincidences are very doubtful and it is improbable that such very faint lines would be represented, while the neighboring line 4327.3, intensity 3, is wanting on the photograph.

The oxygen lines of intensity 4 which are wanting, viz.: Neovius 4465.4 and 4467.8 are in a portion of the spectrum which is somewhat over-exposed, and this fact probably accounts for the non-appearance on the plate. Possibly also the relative intensities of the oxygen lines at the temperature and pressure of the atmosphere of 8 Crucis may be different from their relative intensities in the conditions under which Neovius determined the intensities of the air lines (spark spectrum).

There remains, however, not the slightest doubt that all the stronger oxygen lines are present in the spectrum of 8 Crucis, at least between A 4250 and 4575, and this fact requires no further laboratory experiments for its establishment. It is almost equally certain that there is no trace of true nitrogen lines in this spectrum.

The only measured lines of B Crucis near known nitrogen

but it is improbable, although not impossible, that nitrogen lines of intensity (1) should be present while the strong nitrogen line 4507.7, of intensity 6, is absent.

Besides hydrogen, helium and oxygen, the spectrum of ẞ Crucis shows the probable presence of carbon (4267.2) and magnesium (4481.17). These lines were not included in the first measurements, in the former case, because the line could not be distinctly seen with the higher power, in the latter because I was

doubtful of the existence of the line on account of a slight defect in the film at that point, but other negatives confirm its existence. The spectra of B Crucis, B and e Canis Majoris, and probably B Centauri are all practically identical. They all contain the three unknown strong lines

4552.79

4567.09

4574.68

besides the probable magnesium line 4481.17, the lines of hydrogen, helium, the stronger oxygen lines, and the probable carbon line 4267.2.

Further investigations on this class of stars will be subsequently communicated; in the meanwhile I forward also for reproduction a contact positive from a negative of the spectrum of € Canis Majoris taken on March 15, with the single prism, in which the slit has been focused for rays of about λ 4080, and with a comparison spectrum from an oxygen tube, a Leyden jar and air space being introduced in the secondary circuit of the Ruhmkorff coil. This photograph shows the coincidence of stellar lines with the group of three strong oxygen lines, viz.:

and other neighboring oxygen lines beyond the range of the three-prism train. The lines are all displaced towards the red by motion.

On March 15 the Earth by its motion round the Sun was receding from Canis Minoris with a velocity of 17km per second, which agrees with the direction of the displacement of oxygen lines. There has not yet been time to determine the constants of the single-prism spectroscope, but this does not affect the question of the identification of the oxygen lines in the spectrum of Canis Majoris.

The plates of the spectra of 8 Crucis and e Canis Majoris were exposed and developed by my assistant, Mr. J. Lunt.

MINOR CONTRIBUTIONS ANd Notes

CORRECTIONS TO DETERMINATIONS OF ABSOLUTE WAVE-LENGTH.

IN so far as the literature of the subject is at present available to me, it appears that two corrections have hitherto been neglected in determinations of the absolute wave-length of lines of the solar spectrum. These corrections are so small as to be of slight importance in many researches upon the spectrum, including the identification of solar and metallic lines, but may rise to significance in investigations of the motion of stars in the line of sight, which are very exacting in regard to the accuracy of the wave-lengths of both metallic and gaseous comparison lines, solar lines occurring in stellar spectra, and lines of normally gaseous elements found in stars.

The first correction is due to the effect of the eccentricity of the Earth's orbit. The ordinary formula for the component of the Earth's velocity along the radius vector of its orbit is

2 па

V being the mean orbital velocity of the Earth and equaling

T

and II being the longitude of the Sun when the Earth is at perihelion. The values of u will obviously be at a maximum when the Earth is at quadrature, in April and October, and zero at perihelion and aphelion. Introducing the proper values for a, e and T we at once obtain the maximum value, v=0.50 kilometers per second, which may, of course, be directly obtained from the Nautical Almanac. The effect of this velocity in shifting the lines of the solar spectrum would be as follows:

The second correction is the diurnal one, due to the Earth's rotation, habitually applied to the reduction of stellar velocities by all