Transmission of Infra-red Rays and of Radiant Energy. 65 



dispersion and " over-lap " are probably better. He then investigated in 

 order the absorption by the cornea, the aqueous, the lens, and the vitreous 

 of the bullock's eye. In the cornea, when pressed flat, there is almost 

 invariably some degree of cloudiness : this we also have observed, and the 

 cloudiness probably leads to some general absorption throughout the whole 

 range of wave-lengths. Aschkinass found that the general absorption 

 produced by this cloudiness is greater the shorter the wave-length, and 

 diminishes (as one might expect) considerably for the longer waves. There 

 is no reason to suppose that in the normal eye this phenomenon causes any 

 absorption at all (in the condition of glaucoma, however, it probably does) : 

 and Aschkinass, finding absorption bands at l - 00 fju, 1/25 /x, and l - 50 /u,, comes 

 to the conclusion that " the absorption follows the same course as for water." 

 The same result he found, quite definitely and clearly, for the aqueous and 

 the vitreous. With regard to the lens, the proof of the quantitative equality 

 of the absorption to that of water was more difficult, as we have pointed out 

 above. Aschkinass did not, as we have done, immerse the lens in some 

 non-absorbing fluid (CCU) of approximately the same refractive index : he 

 trusted simply to pressing it between two glass plates. He still found, 

 however, that by virtue of its inhomogeneity it continued to act as a lens, 

 and in order to deal with the absolute value of the absorption he had to 

 apply a correction. Qualitatively, identically the same bands were seen as 

 in the case of water, and quantitatively he came to the same conclusion as 

 ourselves, viz., that the absorption of the lens is in no considerable way 

 different from that of water. 



The absorption of radiant heat by water has been known for some time. 

 Julius investigated the absorption of both water and NaCl solution in small 

 thicknesses. Abney,* using both photometric and thermometric methods, 

 investigated very completely the absorption of water up to layers 2 feet 

 thick. He found that water between the sodium lines and 2 - 4 //, had 

 absorption bands with the following maxima 0-580, 670, 0-780, 0"860, 0'970, 



1- 20, l - 45, l - 90; and 2 50 approximately, the first four bands being shallow 

 except for thick layers ; the last five bands being of increasing depth. 

 Paschenf carried out a more complete investigation of the far infra-red 

 absorption up to 10 /j, ; he found that thin layers of water even take up a 

 considerable portion of the incident radiation of wave-length greater than 



2 - 3 ll. Thus a layer 0'03 mm. thick transmitted at no wave-length more 

 than 30 per cent, of the incident energy. A layer 2 mm. thick would, 

 therefore, be totally opaque for wave-lengths greater than 2 -3 fju. This is an 



* ' Eoy. Soc. Proc.,' vol. 35, p. 328. 

 t ' Wied. Ann.,' vol. 52, p. 216 (1894). 



