In the year 1800, and in the same volume of the Philosophical Transactions that contains Volta’s celebrated letter to Sir Joseph Banks on the Electricity of Contact, Sir William Herschel published his discovery of the invisible rays of the sun. Causing thermometers to pass through the various colours of the solar spectrum, he determined their heating-power, and found that this power, so far from ending at the red extremity of the spectrum, rose to a maximum at some, distance beyond the red. The experiment proved that, besides its luminous rays, the sun emitted others of low refrangibility, which possessed great calorific power, but were incompetent to excite vision. Drawing a datum-line to represent the length of the spectrum, and erecting at various points of this line perpendiculars to represent the calorific intensity existing at those points, on uniting the ends of the perpendiculars Sir William Herschel obtained the subjoined curve (fig. 1), which shows the distribution of heat in the solar spectrum, according to his observations. The space A B B represents the invisible, and B D E the visible radiation of the sun. With the more perfect apparatus subsequently devised, Professor Muller of Freiburg examined the distribution of heat in the spectrum and the results of his observations are rendered graphically in fig. 2. Here the area A B C D represents the invisible, while C D E represents the visible radiation.