LIGHT. 



9. Malus, so late as 1810, made the important discovery of the 

 polarisation of light by reflection, and successfully explained the 

 phenomenon by the hypothesis of an undulatory propagation. 

 The theory subsequently received a great extension from the 

 ingenious labours of Fresnel ; and the still more recent re- 

 searches of Arago, Poisson, Herschel, Airy, and others, have 

 conferred on it so great a degree of probability, that it may 

 almost be regarded as ranking in the class of demonstrated 

 truths. " It is a theory," says Herschel, " which, if not founded 

 in nature, is certainly one of the happiest fictions that the genius 

 of man has yet invented to group together natural phenomena, 

 as well as the most fortunate in the support it has received from 

 all classes of new phenomena, which at their discovery seemed 

 in irreconcileable opposition to it. It is, in fact, in all its 

 applications and details, one succession of. felicities ; inasmuch as 

 that we may almost be induced to say, if it be not true, it 

 deserves to be." 



10. Light and heat are so intimately related to each other, that 

 philosophers have doubted whether they are identical principles, 

 or merely co-existent in the luminous rays. They possess 

 numerous properties in common : being reflected, refracted, and 

 polarised, according to the same laws, and even exhibit the 

 same phenomena of interference. Most substances during com- 

 bustion give out both light and heat ; and all bodies, except 

 the gases, when heated to a high temperature, become incan- 

 descent. Nevertheless, there are many circumstances in which 

 they appear to differ. 



A thin plate of transparent glass interposed between the face 

 and a blazing fire intercepts no sensible portion of the light, but 

 most sensibly diminishes the heat. Light and heat are there- 

 fore not intercepted alike by the same substances. Heat is also 

 combined in different degrees with the different rays of the solar 

 spectrum. A very remarkable discovery on this subject was 

 made by Sir William Herschel, which would seem to establish 

 the independence of the heating and illuminating effects of the 

 solar rays. Having placed thermometers in the several prismatic 

 colours of the solar spectrum, he found the heating power of the 

 rays gradually increased from the violet (where it was least) to 

 the extreme red, and that the maximum temperature existed 

 some distance beyond the red, out of the visible part of the 

 spectrum. The experiment was soon after repeated with great 

 care by Berard, who confirmed Herschel's conclusions relative 

 to the augmentation of the calorific power from the violet to the 

 red, and even beyond the spectrum. This discovery of the ine- 

 quality of the heating power of the different rays led to the 

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