It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Sir W. Herschel, that _the different-coloured rays have not the same illuminating power_. The violet rays appear to have the least illuminating effect; the indigo more, and the effect increases in the order of the colours,--the _green_ being very great; between the green and the yellow the greatest of all; the yellow the same as the green; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays to _heat_ bodies, varied with their power to illuminate them. He introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the Indigo than when it was placed in the violet, still higher in blue, and highest of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all; and concluded that _rays proceed from the sun, which have the power of_ HEATING, _but not of illuminating bodies_. These rays have been called _invisible_ solar rays. They were about half an inch from the commencement of the red rays; at a greater distance from this point the heat began to diminish, but was very perceptible even at the distance of 1-1/2 inch. He determined that the heating power of the _red_ to that of the _green_ rays, was 2-3/4 to 1, and 3-1/2 to 1, in red to _violet_. He afterwards made experiments to collect those invisible calorific rays, and caused them to act independently of the light, from which he concluded that they are sufficient to account for all the effects produced by the solar rays in exciting heat; that they are capable of passing through glass, and of being refracted and reflected, after they have been finally detached from the solar beam.