and the other an instant after, the beginning of totality have been called the 'cusp' and the 'flash' spectrum. A similar pair occur, of course, when totality ends, but in reverse order. Figure 1 shows an enlargment of a portion of the cusp spectrum at third contact. This photograph was made by Professor E. B. Frost, of the Yerkes Observatory, at the eclipse of 1900. It furnishes an opportunity to compare directly the dark lines of the ordinary solar spectrum with the bright lines of the chromosphere. It is of the greatest interest to learn whether the two series of lines are identical, in whole or in part, though reversed, and in any case to study the characteristics of these bright lines. This photograph was made about ten seconds after the end of totality. The thin line of the photosphere, which had then emerged from behind the moon, was drawn out by the prism into the bright band, which constitutes the larger portion of the picture. This is the ordinary solar spectrum. It will be noted that while in spectra as usually seen the lines are straight, since a straight slit is used, here the lines are arcs, since nature furnishes a crescent of light. An examination of these dark arcs shows that in nearly all cases they become bright lines at the upper edge of the spectrum. This 'reversal' is due to the fact that just beyond the point where the crescent of sunshine ceased, was a small extension of the chromosphere, which was not covered by the moon. The precise determination of all the facts, which this and other similar photographs teach, is one of the important problems of total eclipses. The problem is somewhat complicated, as pointed out by Professor Frost; for although few dark arcs can be seen which do not terminate in a bright tip the curvature and position appear to be slightly different in some cases for the bright lines. Figure 2 shows the 'flash' spectra made at the second contact, that is, at the beginning of totality. The sun is entirely hidden by the moon, and all the lines which appear are doubtless due to the chromosphere. Certain irregularities, or 'bunches,' in the arcs, however, are due to solar prominences. From an examination of these and other photographs Professor Frost has measured and identified several hundred lines, and has reached the following conclusions: "At least 60 per cent, (and probably many more) of the stronger dark lines of the solar spectrum are found to be bright in a stratum not exceeding (for the majority of the lines) 1″, or less than 500 miles in height above the solar photosphere. There is moreover no reason in general to suppose that this is not equally true of the fainter lines. Therefore we may regard the existence of a reversing layer at the base of the chromosphere as fully confirmed by the photographs." These results are especially important since they contradict to some extent those which have been previously obtained. While the elevation of the strata which produce the most of the lines is less than 500 miles, the height of other gases
Page:Popular Science Monthly Volume 60.djvu/253
This page has been validated.
TOTAL ECLIPSES OF THE SUN.