Page:Popular Science Monthly Volume 4.djvu/409

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spectrum will stretch athwart the field of view like a scarlet ribbon, with a darkish band across it, and in that band will appear the prominences, like scarlet clouds; so like our own terrestrial clouds, indeed, in form and texture, that the resemblance is quite startling: one might almost think he was looking out through a partly-opened door upon a sunset sky, except that there is no variety or contrast of color; all the cloudlets are of the same pure scarlet hue. Along the edge of the opening is seen the chromosphere, more brilliant than the clouds which rise from it or float above it, and for the most part made up of minute tongues and filaments.

Fig. 4.
PSM V04 D409 Spectroscopic aspect of a prominence.jpg
Spectroscopic Aspect of a Prominence.

If the spectroscope is adjusted upon the F line, instead of C, then a similar image of the prominences and chromosphere is seen, only blue instead of scarlet; usually, however, this blue image is somewhat less perfect in its details and definition, and is therefore less used for observation. Similar effects are obtained by means of the yellow line near D, and the violet line near G. By setting the spectroscope upon this latter line and attaching a small camera to the eye-piece, it is even possible to photograph a bright protuberance; but the light is so feeble, the image so small, the time of exposure needed so long, and the requisite accuracy of motion in the clock-work which drives the telescope so difficult of attainment, that thus far no pictures of any real value have been obtained in this manner.

Prof. Winlock and Mr. Lockyer have attempted, by using, instead of the ordinary slit, an annular opening, to obtain a view of the whole circumference of the sun at once, and have partially succeeded. Undoubtedly, with a spectroscope of sufficient power, and adjustments delicate enough, the thing can be done; but as yet no very satisfactory results appear to have been reached. We are still obliged to examine the circumference of the sun piecemeal, so to speak, read-