Page:The wonders of optics (1869).djvu/109

 it was for a long time supposed that the heat-giving properties of any part were in direct proportion to the amount of its luminous effect; but Sir John Herschel proved by a long series of experiments that the heat of the spectrum gradually increased from the extreme violet to the extreme red, and that passing this point it still further increased until it attained its maximum at a point where not a single ray of light existed. From these grand experiments he adduced the important conclusion, that in solar light there existed invisible rays, which produced heat, and which possessed even a less degree of refrangibility than the extreme red rays. Sir John Herschel then tried, but unsuccessfully, to determine the exact refrangibility of the invisible heat rays.

Sir Henry Englefield compared these results, and obtained the following figures:

Bérard obtained similar results, but he at first found that the maximum of heat was just at the end of the extreme red, and that beyond it the air was only about one-fifth warmer than the ordinary temperature. Sir John Herschel attributed these discordant results to Bérard having used a thermometer with too large a bulb; he accordingly repeated his experiments with other instruments with long narrow bulbs, and arrived at similar results to those obtained by the English philosopher.

We will now pass on to the physical properties of the other end of the spectrum. Towards the end of the last century, Scheele, a Swedish philosopher, remarked that