Biographical Memoir of' Sir William Herschel 11 



Tracts unequally, and divides into coloured elements, contains, 

 also, an invisible heat, whose action may be rendered sensible 

 and may be measured. 



The same observer further proposed to himself, to discover 

 what are the rays which possess the power of illuminating bo- 

 dies in the highest degree. He found, by a particular set of 

 experiments, that this property belongs to the yellow rays, and 

 that it diminishes with considerable rapidity, as we pass from 

 these rays to either extremity of the spectrum. 



These singular discoveries excited a lively interest in all the 

 learned societies. The existence of an invisible radiating heat, 

 mingled with the solar light, was disputed. The discoverer was 

 himself exposed to contradictions which exceeded all the bounds 

 of literary criticism ; but that great philosopher having given 

 the necessary explanations, kept silence on the subject. His ex- 

 periments were repeated in England, Germany, and France, 

 under the eyes o^^the most expert observers in Europe, and the 

 truth of the results was universally recognised. 



It happened, also, that the distinction of the coloured rays, 

 and of the invisible heat which the sun transmits, gave rise to the 

 discovery of another not less remarkable property of the light of 

 that star. The intensity of the chemical action of the different 

 rays was made the subject of observation, and it was found that 

 this action also, like that of the heat, subsists in an unilluminated 

 space, but at the opposite extremity of the spectrum beyond the 

 violet rays. We merely mention this experiment, as it does not 

 properly belong to our subject ; and it is enough for us to add, 

 that, at the present day, the existence of invisible rays of heat 

 mingled with the sun''s light, can no longer be questioned. It 

 was chiefly in this that the discovery announced by Herschel 

 consisted. It seemed as if he were destined to discover and 

 render sensible objects and properties, which had eluded the re- 

 search of all other observers for a long series of affes. 



Although our planetary system occupies an extent of 

 twelve hundred millions of miles, it may yet be said to form but 

 an imperceptible point in the immensity of space. Thus far has 

 the genius of man enabled him to penetrate into the vast regions 

 of the universe. He has seen innumerable suns beyond the na- 

 tural limits of his senses ; for the divine intellect frtnn which his 



