10 Biographical Memoir of' Sir William HerscheL 



In tracing the origin of this question, we find it in the writ- 

 ings of a celebrated woman, whose name belongs to the literary 

 history of France. Emilie du Chatelet, previously to her trans- 

 lating and commenting upon the works of Newton, had sent a 

 physical memoir to the Academy of Sciences at Paris, and af- 

 terwards embarked with Euler in the examination of one of the 

 greatest objects of natural philosophy, the theory of fire. In 

 this memoir of Madame du Chatelet's, which was printed in 

 1738 by order of the Academy, the illustrious author proposes y 

 to collect a sufficient quantity of homogeneous light to prove 

 whether the differently coloured primitive rays have not also 

 unequal degrees of heat ; whether, as appears to her to be very 

 probable, the red ray, for example, does not give more heat than 

 the violet ray. The writer adds, " the experiment deserves 

 to be tried by those philosophers who may examine this es- 

 say." The idea here expressed was proved correct, as we have 

 said, by the observations of Landriani and Bochon. Herschers 

 experiments on the same subject not only afforded a complete 

 solution of the question, but led to entirely new results. He 

 measured with precision the thermometrical effects of the seven 

 unequally refrangible rays, and found that the red rays con- 

 tained of themselves more heat than all the others together. 

 The impression on the thermometer rapidly diminishes from 

 the red to the violet rays, which are placed at the other extre- 

 mity. The principal feature of HerscheFs talent was an ex- 

 traordinary disposition to consider the same object with unre- 

 mitting perseverance, and under every point of view. On re- 

 peating his experiments on the solar rays, he wished to deter- 

 mine the limit at which all sensible impression of heat ceases, 

 and the point at which the impression is strongest. While en- 

 gaged in this investigation, he met with a very unexpected re- 

 sult ; he saw that the thermometrical effect continues beyond 

 the red rays in the dark space bordering upon the spectrum, 

 and it was even in that unilluminatcd space, and upon the pro- 

 longation of the axis, that he found the point where the heat 

 communicated is the greatest. The situation of this point is 

 found to vary according to the circumstances of the experiment ; 

 but, be this as it may, it is certain that this mixture of rays 

 which the same star transmits to us, and which the prism re- 



