The Ultra-Violet Region in Sun-spot Spectra.
By John Evershed.

It is well known that the more conspicuous of the lines which are intensified or otherwise affected in the spectra of sun-spots are found in the less refrangible region of the spectrum, those in the red being by far the most easily seen, whilst in the ultra-violet the lines of the same elements, although far more numerous in this region, seem to be totally unaffected.

A considerable number of photographs of sun-spot spectra, taken in the ordinary way, in the region more refrangible than G, have from time to time been examined in detail by the writer, but, excepting always the hydrogen lines and H and K, these have always shown an entire absence of details which could be certainly distinguished from the adjacent photosphere spectrum.

Recently, however, with improved apparatus and exceptionally favourable atmospheric conditions, a series of spectra have been obtained at this observatory showing a considerable number of affected lines in the region A 3990-4350.

Owing to the intrinsic darkness of the spot band in the ultraviolet, a very long exposure is necessary, in photographing the umbral spectrum, to give a density equal to that of the spectrum of the surrounding photosphere. Thus I have found for the region near K the umbra requires from ten to twelve times the exposure needed for the photosphere, and it is obvious that in long exposures the diffuse light from the sky will impress the plate more or less, superposing the general solar spectrum on the true spot spectrum. It is questionable, however, whether this admixture of skylight is sufficient to explain the apparent poverty of detail in the ultraviolet, as compared with the red and yellow regions of the spot spectrum.

The photographs were taken in the fourth order of a Rowland plane grating, having 14,438 lines to the inch and a ruled surface of 3 inches. The collimator used is a visually corrected lens of 2 inches aperture and 36 inches focus. The camera lens is a plano-convex of 4-in. aperture and 7-ft. focus for H. The slit is provided with a sliding shutter, having a <-shaped aperture, which enables its length to be varied within wide limits during an exposure. In photographing a spot spectrum, the slit is reduced to a length equal to about two-thirds or less of the diameter of the umbral image. A long exposure is then given, after which the slit is lengthened, and the exposure continued for a short interval to impress the adjacent photosphere spectrum.

The spectrograph was erected in the spectroheliograph room, using as image lens the 12-in. photo-visual lens usually employed for the spectroheliograph. The exposures given on spot and photosphere were as follows:

The first set of three exposures represents the spectrum of the large southern spot, which passed the central meridian on June 21. The second series represents the same spot after a rotation of the Sun. The latter was not obtained under nearly as favourable conditions as the former, and probably many more affected lines will be added to the list in subsequent photographs.

The two plates were examined and measured quite independently; and in the region where they overlap, the agreement in the estimates of intensity was remarkably good, although, as was to be expected, fewer affected lines were noted on the July plate.

In the following table I give only those lines in the spot spectrum which can be quite certainly distinguished from the photosphere spectrum by an increase or decrease of intensity, or by the absence of a line in the one spectrum which is present in the other. Since a considerable number of the lines visible in the umbral spectrum could not be traced on the photosphere spectrum, the positions of these were measured with a micrometer with respect to neighbouring known lines, from which the wave-lengths were deduced. The measured wave lengths are given in the first two columns to two places of decimals only, to distinguish them from those lines which could be identified with solar lines by inspection to these Rowland's values of the wave-lengths are given. The third and fourth columns give the estimated intensities in photosphere and umbra respectively, the former being from Rowland's table. In estimating intensities the spot line is compared with one or more of the neighbouring solar lines having the same apparent intensity, and the intensities of these comparison lines are taken from Rowland's table after comparison with Higgs'

map.

:

It is to be noted that, as in the visible region of the spectrum, titanium and vanadium figure largely among the strengthened lines, whilst of the seven affected iron lines six are weakened in the spot. The hydrogen lines y and 8 are notably weakened, both in width and intensity. Both are displaced relatively to the photosphere lines, but, curiously enough, in opposite directions, y 05 towards the red on the July plate, & the same amount towards the violet on both plates.

List of Lines affected in Sun-spot Spectra in the region between HS and Hy.

List of Lines affected in Sun-spot Spectra, etc.-continued.

Intensity Intensity

Remarks.

in Element. Umbra.

The Spectrum of Comet 1907 d (Daniel). By John Evershed.

Notwithstanding the very unfavourable conditions prevailing here during the south-west monsoon, attempts were made to photograph the spectrum of this comet on several occasions in August and September. Owing to the short intervals of time practically available for making exposures on the comet (three-quarters of an hour at most), it seemed necessary to use a spectrograph having the greatest possible light-efficiency. I therefore determined to try whether an objective prism spectrograph of short focal length could be successfully employed. On account of the almost constant presence of cloud, more or less thick, in the eastern sky, the results do not come up to expectation, but are perhaps of sufficient interest to place on record.

A prismatic camera was constructed with the two very perfect 60° prisms, of specially selected crown glass, which were successfully used at recent eclipses. The prisms have an effective aperture of 42 mm., and the photographically corrected lens has a focal length of about 600 mm. The instrument was attached to the 6-in. Cooke equatorial in the south dome of this observatory, the plane of dispersion being made parallel to a circle of declination. For guiding, a straight-edged bar was placed in the eyepiece of the finder attached to the equatorial, and this was made parallel to the diurnal motion. In making an exposure, the comet's nucleus was kept on the edge of the bar, and nearly in the centre of the field (defined by a cross-wire), by means of the slow-motion handles of the equatorial.

In the course of several weeks, about the time of the comet's maximum brightness, only three fairly favourable opportunities occurred. These were August 28, September 3, and September 15. The best result was obtained on the first-named date, with an exposure of half an hour. On September 3 the comet was obscured by cloud for about half the available time before daylight, and during this interval the opportunity was taken to photograph the spectrum of Procyon on the same plate, the star being brought to the edge of the bar in the eyepiece, but at a little distance from the centre of the field. In the plate exposed for twenty minutes on September 15, under apparently favourable conditions, no trace of the spectrum can be seen. This is probably due to the low altitude of the comet, for the spectrum of Regulus, then about 3° N. of the comet, is only faintly impressed on the plate. Wratten's "Drop Shutter" plates were used in all the trials.

In the spectrum of August 28 the most striking features shown are a pair of extremely bright lines in the ultra-violet, corresponding with the nucleus of the comet, *aud a series of monochromatic images of the tail, having the following approximate wave-lengths :▲ 358, 369, 378, 401, 426, 455.

Owing to a slight drift of the comet in R. A., the images of the nucleus

are linear.