246 SPECTKUM erful electric light is eight times that of the vis- ible." Tyndall, with the more efficient means at his command, repeated Herschel's experi- ments on the condensation of the invisible rays, and caused them to ignite to whiteness solids like platinum. This property of these con- densed rays he called " calorescence." Seebeck (Memoires de Vacademie, Berlin, 1819) first showed that the position of maximum he.at in the spectrum changes with the nature of the prism, and sometimes occurs in the red. Mel- loni (Journal de Vinstitut, vol. i., p. 212) proved that the effects observed by Seebeck were owing to the absorptive action of the materials of the prisms, and with prisms filled with water and alcohol he observed the maximum temper- ature in the yellow. In a spectrum which Mel- loni obtained by passing the sun's rays through a prism of rock salt (the most diathermanous of all substances), he found the maximum of heat beyond the red rays at a distance from the line B nearly equal to the distance of this same line B from the line F. Melloni used the thermo-battery of Nobili for a thermometer. Sir John Herschel ("Philosophical Magazine," April, 1840) examined the distribution of heat in a spectrum by using paper covered on one side with lampblack and then moistened with ether A B < or alcohol. On allowing the spectrum to fall on the uncoated side of the paper, he observed the rate of evaporation of the ether, and thus saw the superior heating ef- fect of the rays beyond the red; and by this meth- od he also detected the existence in the invisible spectrum of ather- mic bands, which corresponded to the Fraun- hofer lines seen in the visible spectrum. In 1843 Dr. J. W. Draper of New York obtained photographs of these bands by projecting a spectrum on a daguerreotype plate, while the latter was at the same time exposed to a dif- fused light of feeble intensity. In 1847 Fizeau and Foucault, by means of minute mercurial thermometers, detected in the spectrum of a flint glass prism a large athermic band be- yond the red, at a distance from the line A equal to the distance of this line from D. In 1871 Lamansky, with a linear thermo battery (" Philosophical Magazine "), confirmed the ob- servations of Herschel. The most recent re- search on the distribution of heat in the spec- trum is by Dr. J. W. Draper ("American Jour- nal of Science," 1872). He maintains that the observed increase of heat in the spectrum, as we proceed to its red end and advance beyond this point, is owing to the fact that any prism by its unequal refractive action on the spectral rays must give a spectrum which is abnormally condensed at its red end and dilated at its vio- let. He calls attention to the fact that the middle of the normal or diffraction spectrum is at the point where falls the ray whose wave length is -0005768 of a millimetre. This is a point a little above the line D. The distri- bution of the rays in the prismatic spectrum of a flint glass prism compared with the normal spectrum is shown in fig. 8, where the two spectra have the same length, and their optical centres, as given by wave length, are in the same line. From a long series of experiments on the spectra obtained by prisms of flint glass, rock salt, carbon disulphide, and quartz, Dr. Draper infers that the amount of heat con- tained in the visible normal spectrum from its optical centre to the line H a is equal to the heat contained in the same spectrum from its opti- cal centre to the line A. " Assuming this as true," he says, " it necessarily follows that in the spectrum any two series of undulations will have the same heating power, no matter what their wave lengths may be." It appears that this conclusion is too extended a deduction from such a restricted result as Draper reached ; for if the variations of heat in the spectrum were symmetrically divided by a line drawn through its optical centre, the same result would be attained by Draper's method of ex- perimenting. In other words, if the maxi- 1
- B C D EFGH
FIG. 8. mum or minimum of heat existed at the optical centre, and the heat declined uniformly above and below this point, or if a series of maxima and minima were symmetrically distributed above and below the optical centre, then each half of the spectrum, divided at its optical centre, would give the same heating power. Recently E. Lundquist (Poggendorff's Anna- len, vol. civ.) has shown how Cauchy's for- mula, which serves to connect the index of refraction of a ray with its wave length, may lead to an expression which serves to reduce the distribution of heat observed in a pris- matic spectrum to what it would be in a nor- mal spectrum. He finds that the observations of Lamansky, made with flint glass and rock salt prisms, when thus reduced place the maxi- mum of heat about the middle of the normal spectrum, and the heat diminishes uniformly on both sides of this point. In the spectrum of the electric light, however, the maximum of heat is near the line A ; hence in this case the optical centre of the spectrum does not divide it into two portions having equal heat- giving powers. Chemical Actions of the Spec- trum. Conclusions as to the distribution of chemical action in the spectrum have generally
