The Sun 



computation applies solely to the luminous 

 layer, which alone sends radiations to us. We 

 can only conjecture that the sun's temperature 

 decreases in general from the centre to the 

 periphery. 



Our knowledge of the sun owes its chief 

 progress to the direct telescopic study of the 

 luminary's surface. As long as we are satisfied 

 to look at it with the naked eye or through a 

 coloured glass its disc appears uniformly bright, 

 scarcely less at the edges than at the centre, 

 and on rare occasions a spot of unusual size can 

 be distinguished. 



It was only with the help of the telescope 

 that our investigations could be carried further, 

 and this immense improvement in our means of 

 investigation dates from Galileo, who by the 

 conjunction of two lenses invented, or reinvented 

 after the Dutch opticians, the terrestrial and 

 astronomical telescopes. With these instru- 

 ments, which were then very imperfect, and of 

 low magnifying power, Galileo gave fresh 

 impulse to the science of solar physics, which 

 finally cost him his eyesight. 



159 



