Masterpieces of Science 



in comparatively recent times that the hypo- 

 thesis has been finally established. For even 

 within the last ten years a theory was put for- 

 ward which accounted satisfactorily for most of 

 the changes of appearance observed in the spots, 

 by supposing them to be due to solar clouds 

 hanging suspended at a considerable elevation 

 above the true photosphere [the luminous en- 

 velope surrounding the sun]. 



Sir William Herschel, reasoning from terrestrial 

 analogies, was led to look upon the spot cavities 

 as apertures through a double layer of clouds. 

 He argued that were the solar photosphere of 

 any other nature, it would be past comprehension 

 that vast openings should form in it, to remain 

 open for months before they close up again. 



Whether we consider the enormous rapidity 

 with which the spots form and with which their 

 figure changes, or the length of time that many 

 of them remain visible, we find ourselves alike 

 perplexed, unless we assume that the solar photo- 

 sphere resembles a bed of clouds. Through a 

 stratum of terrestrial cloud openings may be 

 formed by atmospheric disturbances, but while 

 undisturbed the clouds will retain any form once 

 impressed upon them for a length of time corre- 

 sponding to the weeks and months during which 

 the solar spots endure. 



And because the solar spots present two dis- 

 tinct varieties of light, the faint penumbra and 

 the dark umbra or nucleus, Herschel saw the 

 necessity of assuming that there are two beds of 

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