109 a.]

SPECTROSCOPIC OBSERVATIONS OF THE SUN.

FIG. 86 h.

209

stand immediately over the black Fraunhofer's lines; on some occasions Mr. Lockyer has noticed an alteration of wave length when examining the prominences, and on March 14, 1869, he obtained evidence of a cyclone on the sun's limb. At one edge of the prominence the green line was thick and a little less refrangible than the black line F, showing that the hydrogen emitting this light was receding from the earth (fig. 86 e). At the centre of the prominence the green line was very thick, and broader at the top than at the base (fig. 86ƒ), and at the other side of the prominence the line was more refrangible than the black line, indicating the approach of the hydrogen towards the earth (fig. 86 g). The examination of sun-spots by this method has led to several curious observations. When the image of a spot is allowed to fall on the slit of the spectroscope the solar spectrum is seen with a dark band traversing its whole length from red to violet. This band is dark, not black, indicating that some light, but very little compared with that given off by the photosphere, is emitted by the spot, and thus showing that the spot is a region of general absorption. But this is not all; at those places where the black Fraunhofer's lines cross the dark band, these lines are frequently thickened-the sodium lines for instance (fig. 86 h), though this is not the case with all the lines, as some pass across the dark band without any sensible alteration: this shows us that the spot is not only a region of general but also of selective absorption. We have already seen that Frankland and Lockyer have found that a gas in a very rarefied condition gives a spectrum consisting of thin lines, but as the pressure increases these lines widen out and the spectrum may even become continuous. They have also noticed that when light from an ignited solid, and which exhibits a continuous spectrum is passed through an absorbing medium, the lines are thin when the density of the absorbing medium is low, and thicken as the density is increased. This phenomenon is well seen by placing in front of the slit of the spectroscope a hard glass tube containing a small quantity of sodium, and from which the air has been exhausted before sealing: when the light from an incandescent solid, such as lime ignited by the oxyhydrogen blow-pipe, is passed through the instrument, the sodium lines are often seen superposed on the continuous spectrum, on gently heating the tube the sodium is volatilized and the bright lines are gradually replaced by black ones; as the density of the sodium vapour increases the lines widen out considerably. These experiments show that the sun-spots are regions where the cool absorbing medium has a greater density than that above the photosphere, and this increase of density is due to the fact that the spots are depressions in the photosphere containing cool absorbing vapours. In the neighbourhood of the spots the spectroscope often reveals the presence of prominences containing the same constituents as those on the limb, and frequently the black Fraunhofer lines are completely blotted out and sometimes replaced by bright lines which are visible above the brightness of the underlying photosphere. In some cases the motion of these prominences is indicated by a change of the FIG. 86 i. FIG. 86 m.

FIG. 86 k.

FIG. 86 l.

III

refrangibility of the lines, and occasionally the gases are being projected with. such rapidity from the sun that the bright lines lie on the more refrangible side of the absorption lines (fig. 86 i). As the prominences are usually seen in the

210

SPECTROSCOPIC OBSERVATIONS OF THE SUN.

[109 a. neighbourhood of spots, and generally on the preceding edge of them, it would seem that the spots may be caused by the projected gases cooling and returning to the sun and thus increasing the quantity of absorbing medium. Occasionally the motion of the absorbing medium is rendered apparent by the alteration of the position of the black lines (fig. 86 k): when this happens the absorption line appears to thicken, but on one side only; sometimes it is even seen quite double. Figs. 867 and 86 m.

The spectra of all the metals seem to contain lines some of which are shorter than others. This difference of length may be observed in several ways and is of great importance. One method is to pass sparks between poles of the metal under examination, and by means of a lense to throw an image of the sparks on to the slit of the spectroscope; it is, however, necessary to vary the interval between the poles when dealing with bodies of different volatilities. On examining a spectrum in this way some lines are found to pass from one pole to the other; these are generally the air lines from the incandescent oxygen and nitrogen between the electrodes. If the poles are too close together some of the metallic lines may extend across the interval, and in the case of yellow sodium lines this is invariably the case, these being the longest lines yet observed in other cases, however, the lines only extend to a certain distance from the poles, the metal magnesium for example gives seven lines, three of which (those which when reversed in the solar spectrum constitute b) are longer than the other four. If the electrodes are enclosed in a tube from which the air can be gradually removed, it is found that as the exhaustion proceeds the shortest line first vanishes; on continuing to abstract the air the spectrum becomes more simple, and may in some cases be reduced to a single line. The lines disappear in a constant order, the shortest lines first vanishing and the longest remaining. When the spectrum of nitrogen is observed at very low pressures it is found to consist of a single line in the green: this is the longest line of nitrogen and it is the one observed by Huggins in the spectra of some of the nebulæ (109 note). This simplification of the spectrum may also be effected by the dilution of the metal either by combining it with other metals or with non-metallic elements. Thus the spectra of the metallic chlorides contain only the longest metallic lines, the shorter ones being entirely absent; only the three longest lines of magnesium are seen in the spectrum of its chloride. When the spectra of alloys are examined, it is found that the longest lines of both metals are present, and a small variation of composition causes a lengthening of the lines of the metal, the percentage of which is increased. A quantitative process of assay of alloys of copper and gold by this method has been suggested by Lockyer and Roberts (Phil. Trans. 1874, 495). It frequently happens that in the solar spectrum only some of the metallic lines are represented by Fraunhofer's lines; when this is the case it is always the longest lines that are reversed: for instance, only the two long lines of aluminium are found in the spectrum of the sun, the other ten being absent. (Lockyer, Phil. Trans. 1873, 253.)

When the electric arc obtained from a powerful battery is passed between metallic electrodes placed in a horizontal position, and the image of the arc is projected by means of a lense on the vertical slit of the spectroscope, the spectrum of a section of the arc is obtained, in which the long lines extend to a greater distance from the centre of the arc than the others. Lockyer has photographed a number of the more refrangible portions of these spectra with great success (Phil. Trans. 1874, 479). Fig. 86 n shows the long and short lines of manganese from one of these photographs. By placing a shutter before the slit of the instrument exposing only a portion of the slit at one time, a number of photographs of different spectra can be obtained on the same plate, and the most rigid comparisons may thus be made. This process has far

109 a.]

SPECTROSCOPIC OBSERVATIONS OF THE SUN.

211

extended the knowledge of the constitution of the sun. (Lockyer, Phil. Trans. 1874, 479.)

In fig. 86 o the coincidences between

some of the Fraunhofer lines and the bright lines of the iron spectrum are

FIG. 86 n.

212

CHANGE IN THE REFRANGIBILITY OF LIGHT,

[109 a. well shown, and fig. 86 p is a comparison of the solar spectrum with the spectra of calcium on the left and aluminium on the right. It will be seen that the calcium employed contained aluminium, and the aluminium contained calcium, but the characteristic lines of each metal are evident by their thickness. The H

FIG. 86 p.

lines in the solar spectrum near the top of the figure are calcium lines, and are coincident with the thick lines in the left hand spectrum, the corresponding lines in the right hand one, where the calcium exists only as an impurity, being thin. The figures are copied from photographs kindly supplied by Mr. Lockyer. In the three preceding figures the upper end is the more refrangible.

(110) Change in the Refrangibility of Light-Fluorescence. A remarkable discovery was made by Stokes, whilst engaged in pursuing the observations of Sir J. Herschel respecting the effect of light upon an acid solution of quinine sulphate. This liquid is colourless when viewed by transmitted light, but if placed in a glass and looked at from above, it exhibits, when exposed to direct light, a beautiful and intense blue upon its front surface; but the light which has passed through one vessel containing the sulphate exhibits no such appearance on the front face of a second vessel of the liquid which is similarly exposed to it. Now the rays which produce this beautiful blue colour are not the ordinary blue rays, but those of the most refrangible portion of the spectrum, which, under ordinary circumstances, are not perceptible to the eye, but which are remarkable for their powerful chemical action,

110.]

OR FLUORESCENCE.

213

and show their presence by their chemical effect upon a surface covered with argentic chloride, or with some other photographic preparation.

Stokes (Phil. Trans. 1852, 468) found that when a tube filled with an acid solution of quinine sulphate was placed successively in each of the colours of the prismatic spectrum, 'throughout nearly the whole of the visible spectrum the light passed through the fluid as it would have done through so much water, but on arriving nearly at the violet extremity, a ghostlike gleam of pale blue light shot right across the tube. Ou continuing to move the tube, the blue light at first increased in intensity, and afterwards gradually died away. It did not, however, cease to appear until the tube had been moved far beyond the violet extremity of the spectrum visible on a screen.' On examining by a second prism the dispersed light thus obtained, it was found that it contained rays extending over a considerable range of refrangibility within the limits of the visible spectrum; but the least refrangible rays, or those of the red end of the spectrum, were wanting.

This power of changing the refrangibility is by no means uncommon, especially amongst organic substances, most of which show it in a degree more or less marked. The change is not confined to the invisible rays, but extends also to those already visible, the more refrangible being generally the most affected, though it is not limited to this portion of the spectrum, for with an alcoholic solution of chlorophyll the effect is seen to commence in the red rays. It is, however, to be remarked, that as yet, in every instance, the altered ray gives rise to others which are less refrangible. The change is never to rays of greater refrangibility.

Stokes has found (Journ. Chem. Soc. 1869 [2], vii. 174) that the fluorescence of an acid solution of sulphate of quinine is destroyed by the addition of the following acids of their salts:-Hydriodic, hydrobromic, hydrochloric, hydroferrocyanic, hydropalladiocyanic, hydroplatinocyanic, hydrosulphocyanic, and hyposulphurous. The introduction of oxacids and their salts produces no effect on the fluorescence.

Amongst the bodies in which this phenomenon is most strikingly exhibited may be mentioned, decoction of the bark of the horse-chestnut (which contains the vegetable principle termed esculin), an alcoholic tincture of chlorophyll (the green colouring matter of leaves), tincture of the seeds of stramonium, and tincture of turmeric. Many of these substances, when spread upon paper, form surfaces which, if used as a screen for receiving the solar spectrum, exhibit a prolongation of the more refrangible end far beyond the violet and lavender rays which are visible on ordinary white paper; and with due precaution the fixed lines of this prolongation may be distinctly seen. A slip of ivory makes a very tolerable screen for this purpose. Glass, coloured yellow with uranic oxide, also exhibits