Amongst the most noteworthy results of Professor Langley’s observations in the Sierra Nevada was the enormous extension given by them to the solar spectrum in the invisible region below the red. The first to make any detailed acquaintance with their obscure beams was Captain Abney, whose success in obtaining a substance—the so-called “blue bromide” of silver—sensitive to their chemical action, enabled him to derive photographic impressions from rays possessing the relatively great wave-length of 1,200 millionths of a millimetre. This, be it noted, approaches very closely to the theoretical limit set by Cauchy to that end of the spectrum. The information was accordingly received with no small surprise that the bolometer showed entirely unmistakable heating effects from vibrations of the wave-length 2,800. The “dark continent” of the solar spectrum was thus demonstrated to cover an expanse nearly eight times that of the bright or visible part.[22] And in this newly discovered region lie three-fifths of the entire energy received from the sun—three-fifths of the vital force imparted to our planet for keeping its atmosphere and ocean in circulation, its streams rippling and running, its forests growing, its grain ripening. Throughout this wide range of vibrations the modifying power of our atmosphere is little felt. It is, indeed, interrupted by great gaps produced by absorption _somewhere_; but since they show no signs of diminution at high altitudes, they are obviously due to an extra-terrestrial cause. Here a tempting field of inquiry lies open to scientific explorers. On one other point, earlier ideas have had to give way to better grounded ones derived from this fruitful series of investigation. Professor Langley has effected a redistribution of energy in the solar spectrum. The maximum of heat was placed by former inquirers in the obscure tract of the infra-red; he has promoted it to a position in the orange approximately coincident with the point of greatest luminous intensity. The triple curve, denoting by its three distinct summits the supposed places in the spectrum of the several maxima of heat, light, and “actinism,” must now finally disappear from our text-books, and with it the last vestige of belief in a corresponding threefold distinction of qualities in the solar radiations. From one end to the other of the whole gamut of them, there is but one kind of difference—that of wave-length, or frequency in vibration; and there is but one curve by which the rays of the spectrum can properly be represented—that of energy, or the power of doing work on material particles. What the effect of that work may be, depends upon the special properties of such material particles, not upon any recondite faculty in the radiations.