1813.] different Rays which compose the Solar Light. 163 



phers, had been called in question by others no less skilful, 

 particularly by Mr Leslie. On that account it was important to 

 remove every doubt respecting; them. It was likewise interesting 

 to know if the invisible or almost invisible rays, situated beyond 

 the extremities of the spectrum, possess any other properties of 

 light. For example, if the reflection of them from polished 

 glass can give them that modification w-hich Malus has distin- 

 guished by the name of polarization. M. Berthollet engaged 

 MM. Malus and Bcrard to undertake this double object. The 

 premature death of Malus has deprived us of the great light 

 which he would without doubt have thrown upon the subject, as 

 he had already done upon other parts of optics by his excellent 

 discoveries; but the researches which he had begun, or projected, 

 have not been lost. M. Berard has persued them with great 

 care. He has finished them with the utmost possible exactness) 

 and has presented the results to the Institute. 



M. Berard 5 with respect to apparatus, has had a great advan- 

 tage over the philosophers who have preceded him in these 

 researches. He made use of the heliostate which Malus had got 

 constructed for the philosophical cabinet of M. Berthollet, and 

 by means of that instrument he obtained a ray of solar light 

 perfectly fixed, on which he could make experiments at pleasure. 

 By decomposing this ray wit!, a prism, he obtained an immove- 

 able coloured spectrum. By placing very sensible thermometers 

 in the spaces occupied by the different colours, he was enabled 

 to compare their calorific effects with the utmost certainty. He 

 ascertained their chemical properties by substituting, in place of 

 the thermometers, chemical compounds easily altered. 



He examined, in the first place, the calorific power of the 

 different rays. We know that they are unequal in this respect. 

 M. Rochon, who first observed this inequality, placed themaxi- 

 mum of heat in the yellow ray, where the illuminating power is 

 greatest. Dr. Hcrschell placed it out of the spectrum, and 

 beyond the red ray. The experiments of Berard have confirmed 

 the experiments of Herschell relative to the progressive heating 

 power of the rays from the violet to the red; but he found the 

 greatest heating power at the extremity of the spectrum itself, 

 and not beyond it. He fixed it at the point, where the bulb of 

 the thermometer was still entirely covered with the red ray: and 

 he found that the thermometer sunk progressively, in proportion 

 as the distance of its bulb from the red ray increased. When 

 he placed the thermometer quite beyond the visible spectrum, in 

 the spot where Herschell fixed the maximum of heat, its eleva- 

 tion above that of the ambient air was only ^th of what it had 

 been in the red ray itself. The absolute intensity of the heat 

 produced was likewise less in the experiments of Berard than in 

 those of Herschell. Do these differences depend upon the 



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