TEE TOTAL SOLAR ECLIPSE. 103 



entire critical region west of the sun, and only five or six degrees below 

 it; and for the Egyptian party to cover the whole region east of the 

 sun and only five or six degrees below it. 



Eclipse observation of the sun itself concerns all that lies outside 

 the photosphere and faculse. While the main features of these outer 

 volumes are for the most part quite irregular in form, yet in a general 

 way they lie, going outward from the photosphere, in the order of 

 reversing layer, chromosphere, prominences and corona. 



The reversing layer was discovered at the eclipse of 1870 by Pro- 

 fessor Young. It appears to consist of a thin stratum of incandescent 

 gases, probably between five hundred and fifteen hundred miles in 

 thickness, immediately overlying the photosphere. Its inner bounding 

 surface seems to be quite definite and regular, but its outer surface 

 is certainly not so. The depth of the stratum of vapor for each ele- 

 ment composing it is probably a function of the properties and quantity 

 of the element in question. The reversing layer is cooler than its sub- 

 strata, yet abundantly hot, if isolated from its underlying strata, to 

 produce a spectrum consisting of thousands of bright lines occupying 

 the positions of the dark lines of the ordinary photospheric spectrum. 

 When the moon, at the eclipse of 1870, gradually covered the photo- 

 sphere, the dark-line spectrum lasted until the instant when the photo- 

 sphere entirely vanished, whereupon the reversing layer was isolated, 

 and Young observed the sudden flashing out of its bright-line spec- 

 trum. A bright line apparently replaced each dark line, and lasted 

 perhaps two or three seconds, until the moon entirely covered the re- 

 versing stratum. 



In so complex a spectrum, lasting but a few seconds, visual observa- 

 tions were difficult, and no records of any considerable consequence 

 could be made. The bright-line (flash) spectrum was photographed 

 for the first time by Shackleton at the eclipse of 1896; and several 

 photographs of it were secured at the three succeeding eclipses, but 

 many were defective on account of poor focusing or other cause. They 

 confirm Young's discovery of the reversing layer, which, by the absorp- 

 tion of its cooler gases, introduces the dark lines in the solar spectrum. 

 The lengths of the arcs not covered by the moon also tell us much con- 

 cerning the thicknesses of the vapors of the various elements, and 

 therefore much concerning the structure of the sun at those levels. 

 Additional work, with more powerful instruments, in perfect adjust- 

 ment, is demanded, with a view to securing better quantitative results. 



Photographs of the reversing-layer spectrum, made with two, four, 

 or more seconds' exposure, are integrated effects. Changes taking 

 place during the exposure are lost. For this reason, it would be very 

 valuable if a continuous record of the spectrum at one point on the 

 limb could be secured on a plate moving in the direction of the length 



