Philosophical Transactions of the Royal Society of London
Forsitan et rosea sol alte lampade lucens Possideat multum caecis fervoribus ignem Circum se, nullo qui sit fulgore notatus, iEstiferum ut tantum radiorum exaugeat ictum. Ltjcbet. v. 610*. f i -In the year 1800, and in the same volume of the Philosophical Transactions that con tains V olta's celebrated letter to Sir J oseph B anks on the Electricity of Contact f , Sir W illiam H ekschel published his discovery of the invisible rays of the sun. Causing thermometers to pass through the various
... ough 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 W illiam H erschel 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 M uller of Freiburg examined the distribution of heat in the spectrum * I am indebted to my excellent friend Sir E dmund TTuau for tbis extract, which reads like divination., t Yol. lxx. + Philosophical Magazine, Ser. 4. vol. xvii. p. 242. MDCCCLXVI. b / v 2 PROFESSOR TYNDALL ON CALORESCENCE. * The width of the image was about 0*1 of an inch. t The width of the linear pile was 0*03 of an inch. 4 PROFESSOR TYNDALL ON CALORESCENCE.