Biographical Memoir (yf Sir William Hemchcl. 9 



placed beneath the surface to a certain depth, extremely small 

 it is true, but actually existing. Now, such of these rays as 

 traverse the envelope of the heated mass obliquely, acquire and 

 preserve a peculiar property which can be rendered sensible by 

 experiment ; they are polarized. But if the same mass, instead 

 of being rendered luminous by its proper temperature, is only 

 covered with an extended flame, which is the source of its light, 

 the rays then do not possess this property. 



We have, therefore, been enabled to submit to this singular 

 test the light which the sun sends to us. M. Arago, the author 

 of this beautiful experiment, and by whose labours natural phi- 

 losophy and astronomy have often been enriched, has in fact dis- 

 covered, that the solar rays, even when transmitted obliquely, 

 are not polarized. It is therefore obvious, that, in regard to this 

 point of the question, the opinion proposed by Herschel would 

 be immediately deduced from the latest discovered properties of 

 light. His researches, also, regarding the annual variations of 

 the solar heat have excited the attention of philosophers ; and 

 we shall soon be in possession of more accurate information on 

 this subject. In several countries, and especially at the Royal 

 Observatory of France, it has been resolved to collect and to 

 publish every year accurate observations with respect to the ex- 

 tent, the progress, and disappearance of the solar spots. 



We have now to mention the memorable experiments of Her- 

 schel, which have given a new development to the physical 

 theory of the sun''s rays. In studying the nature of that star, 

 which had become with him a habitual subject of meditation, he 

 employed variously coloured glasses for diminishing the intensity 

 of the light. He thus had numerous opportunities of observing 

 to what degree the interposition of these glasses modified the 

 heat or light. It was not in the nature of his mind to stop at 

 superficial remarks. He therefore undertook a series of varied 

 experiments, and general physics was enriched with new and 

 important facts, which have been fully confirmed by subsequent 

 observations. It had long been discovered that the rays sepa- 

 rated by the prism, and forming the solar spectrum, do not pos- 

 sess the faculty of heating the terrestrial bodies to the same de- 

 gree. This opinion had been verified by experiments made in 

 Italy and France. 



