TOTAL ECLIPSES OF THE SUN. 245 



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, Avhen totality ends, but in reverse order. Figure 1 shows an 

 enlarg'.Mnent of a portion of the cusp spectrum at third contact. This 

 photograph was made by Professor E. B. Frost, of the Yerkes Observa- 

 tory, at the eclipse of 1900. It furnishes an opportunity to compare 

 directly tlie dark linos of the ordinary solar spectrum with the bright 

 lines of the chromosphere. It is of the gTeatest 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 inoon, 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 T^^ill 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 examina- 

 tion 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 irregu- 

 larities, or "Ininches,' in the arcs, however, are due to solar promi- 

 nences. From an examination of these and other photographs Pro- 

 fessor 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, ^^'hile the elevation of the strata which produce 

 the most of tlie lines is less than 500 miles, the height of other gases 



