OP AETS AND SCIENCES. 147 



spectrum on days of different degrees of clearness, and at different 

 seasons of the year, that the watery vapor absorbed the non-luminous 

 rays so largely, and in so different a proportion to the absorption of 

 the luminous rays, that on two days in August, only nine days apart, the 

 non-luminous spectrum extended, in one case, only as far beyond the 

 red as the green was on the other side, and, in the other, a distance 

 equal to the luminous spectrum. He concluded, from his observa- 

 tions, that it is not possible to make quantitative studies, if the sun be 

 used as a source of light, nor if the eye be used in measurements. 



Knoblauch* published in 1863 the results of a series of experi- 

 ments upon the transmission of radiant heat through rock-salt, in 

 which he studied the effects upon the distribution of heat in the 

 spectrum. He concluded from his experiments, that prisms of chemi- 

 cally pure rock-salt allowed rays of heat of all kinds to pass through 

 in equal proportions ; that the maximum of the solar spectrum found 

 by such a prisni fell outside the red, and that the distribution of heat 

 for the luminous part of the spectrum is the same for prisms of rock- 

 salt and of glass. 



Cloudy rock-salt, however, he found, interrupted rays from the sun 

 more than from an Argand burner, and rays from this burner more 

 than from a source heated to 100° C. ; and also that, chemically or 

 mechanically impure rock-salt diffused the light and exercised a selec- 

 tive absorption. He obtained, also, many other results, which are not, 

 however, of special interest here. 



The next experimenter in this field was Desaines,t who used, as a 

 source of heat, pieces of lime and platinum heated to incandescence in 

 a lamp. The prism used was of rock-salt. He found that the 

 maximum was beyond the red rays, and that water absorbed the 

 ultra-red. Comparing the spectrum from platinum with that from 

 the sun, he found that, while the maximum for platinum was l.°15 

 from the line of no dispersion, for the sun it was 0.°46 at 8 a.m., and 

 0.°51 at noon. 



Lamanskyl explored the sun's spectrum with a very narrow ther- 

 mopile, and found in the ultra-red spectrum places of low heat, which 

 he says are similar to the Fraunhofer lines of the luminous spectrum. 

 But these, as we have seen, had already been noticed by Herschel, 

 and were attributed by Melloni to the absorption of the prism. 



Lamansky, however, finds that, so long as the sun is used as a source 

 of heat, these lines appear the same for a prism of bisulphide of car- 



* Pogg. Ann., cxx. t Comptes Rendus, Ixx. 



J Pogg. Ann., cxlvi. 



