146 PROCEEDINGS OF THE AMERICAN ACADEMY 



maximum, his conclusions were substantially the same as those of 

 Melloni. 



In addition, he made some estimates of the wave-lengths of the 

 dark parts of the spectrum by means of an empirical formula, derived 

 from measurements in the luminous spectrum. As a result of these 

 calculations, he found the wave-length of the limit of the dark spec- 

 trum to be 0.00183 mm \ He admits, however, that this value is very 

 inexact, and we shall see further on that it can hardly be considered 

 an approximation, both because the method is so crude, and because 

 the limit of the spectrum cannot be fixed. 



Dissatisfied with his results, he experimented upon the spectrum 

 from a diffraction grating, in which the elongation of the spectrum is 

 proportional to the wave-length. 



Similar experiments had already been made by Draper,* who 

 stated as the result of his observations that the distribution of heat 

 corresponded to the distribution of light and reached its maximum in 

 the yellow. 



Draper's experiments were made with somewhat crude and not very 

 delicate apparatus, and Miiller, in repeating them with a much more 

 sensitive thermopile, found, as Draper had done, that the maximum of 

 the diffraction spectrum fell in the yellow, but he did not find the in- 

 tensities of heat proportional to the light. 



The curve of distribution which he obtained rose very steeply from 

 the violet to the yellow, and then fell gradually and became insensible 

 at a point distant from the red equal to three and one half times the 

 length of the luminous spectrum. 



The reason of the non-coincidence of intensity of the luminous 

 and thermal spectra had been sought by Mellonif (who thought it was 

 due to a coloration of the crystalline lens), and by Brucke,$ Cima, 

 Tyndall, and Jansen. 



But the most complete research seems to have been carried out by 

 Franz,§ who published an article upon the diathermacy of the media 

 of the eye in 1862. In these experiments, he interposed the various 

 media of the eye, placed between plates of rock-salt, in the path of the 

 rays, which were afterward dispersed by a rock-sal t prism, and found 

 that the humors of the eye all absorb the ultra-red rays very largely, 

 and the luminous rays to a much smaller extent. 



He also showed, by measuring the distribution of heat in the sun's 



* Phil. Mag., 1857, xiii. 153. t Pogg. Ann., lvi. 



t Pogg. Ann., lxv. and lxix. § Pogg. Ann., cxv. 



