$ 263, z6 9 j Herschel' s Theory of the Sun 351 



to the inner cloud-layer rendered luminous by light from 

 above. 



"The sun viewed in this light appears to be nothing else 

 than a very eminent, large, and lucid planet, evidently the first 

 < r, in strictness of speaking, the only primary one of our 

 ^stem ; ... it is most probably also inhabited, like the rest 

 t the planets, by beings whose organs are adapted to the 

 peculiar circumstances of that vast globe." 



That spots were depressions had been suggested more 

 than twenty years before (1774) by Alexander Wilson of 

 (ilasgow (1714-1786), and supported by evidence different 

 from any adduced by Herschel and in some ways more 

 conclusive. Wilson noticed, first in the case of a large 

 spot seen in 1769, and afterwards in other cases, that as 

 the sun's rotation carries a spot across its disc from one 

 edge to another, its appearance changes exactly as it would 

 do in accordance with ordinary laws of perspective if the 

 spot were a saucer-shaped depression, of which the bottom 

 formed the umbra and the sloping sides the penumbra, 

 since the penumbra appears narrowest on the side nearest 

 the centre of the sun and widest on the side nearest the 

 edge. Hence Wilson inferred, like Herschel, but with 

 less confidence, that the 'body of the sun is dark. In 

 the paper referred to Herschel shews no signs of being 

 acquainted with Wilson's work, but in a second paper 

 (1801), which contained also a valuable series of observa- 

 tions of the detailed markings on the solar surface, he 

 refers to Wilson's "geometrical proof" of the depression 

 of the umbra of a spot. 



Although it is easy to see now that Herschel's theory was 

 a rash generalisation from slight data, it nevertheless ex- 

 plained with fair success most of the observations made 

 up to that time. 



Modern knowledge of heat, which was not accessible 

 to Herschel, shews us the fundamental impossibility of 

 the continued existence of a body with a cold interior and 

 merely a shallow ring of hot and luminous material round 

 it ; and the theory in this form is therefore purely of 

 historic interest (cf. also chapter xin., 298, 303). 



269. Another suggestive idea of HerscheFs was the 

 analogy between the sun and a variable star, the known 



