356 THE CHEMISTRY OF THE SUN. [CHAP, 



beautiful features of the corona burst out in all their details. 

 In the spectroscope the sudden withdrawal of the continuous 

 spectrum, especially if a tangential slit be used, causes all the 

 lines, the number of which has been visibly increasing for the 

 last ten minutes as we have seen, to be seen alone on a dark 

 background, although they are not intrinsically brighter than 

 they were before. This sudden withdrawal of the luminous 

 background gives rise at times to what has been called a flash. 

 With a radial slit, at this moment the spectrum is fullest 

 of lines for a reason which we shall refer to in the sequel, but 

 the brightest quickly disappear, and the faintest lines which are 

 the most numerous, attain a still greater visibility at the moment 

 of the second sudden decrease to which we have referred, the 

 instant, namely, at which the inner corona is eclipsed. 



Such being the actual conditions of observation during an 

 eclipse, let us now turn back to chapter xxii., in which the 

 solar atmosphere on the new hypothesis is considered. 



I have now to show in the first place that this gradual un- 

 veiling of that atmosphere enables us to apply a test of the 

 utmost stringency. If we turn back to figure 104, on the new 

 hypothesis stratum A will be hotter than B ; B hotter than 0, 

 and so on ; and different molecules exist in these different strata. 



Now at this point a very important consideration comes in, 

 for if the hypothesis be correct we can predict the locus of any 

 particular spectral variation by considering temperature alone. 



It was stated (p. 306) while discussing the conditions of obser- 

 vation, that whether we were dealing with strata of substances 

 extending down to the sun or limited to certain heights, the 

 spectral lines would always appear to rest on the solar spectrum, 

 and that the phenomena would in the main be the same. 



But though they would be the same in the main, there would 

 be a difference, and this supplies us with a test between the rival 

 hypotheses of the greatest stringency. The stratum B, being 

 further removed from the photosphere than the stratum A, will 



