and Unmeasured Wave-lengths* 403 



progression in the direction of the shorter wave-lengths of 

 the position of the maximum ordinate, as the temperature 

 rises. These results, both in the table and as represented in 

 the plates, are given as preliminary, not as final, for we hope 

 soon to be able to offer other and more exact ones, deduced 

 from the heat-spectra of bodies at all temperatures between 

 that of melting platinum and melting ice. We are entitled, 

 however, even at present, to draw the following conclusions, 

 which are of special interest in connection with the spectra 

 of dark bodies, of which almost nothing has been hitherto 

 known. 



(1) The heat represented by the areas of these curves is 

 almost altogether of a character not observed in that of the sun, 

 these wave-lengths, in general, not being transmissible by 

 glass, which is comparatively permeable to the lowest solar 

 heat-waves that penetrate our atmosphere *. To show this 

 more clearly we have drawn the solar spectrum given by the 

 rock-salt prism in its true position (though not in its true 

 amount) relatively to that of the heat-curves cited. The 

 maximum of the latter lies in every case, it will be seen, far 

 below the very lowest part of the solar invisible heat. 



(2) In spite of the compression of the infra-red by the 

 prism, these heat-curves extend almost indefinitely in the 

 direction of the smaller indices, so far that we can, in fact, 

 represent only a part of this extent in our plate. The measures 

 already cited in case of the curve for the Leslie cube at 178°, 

 for instance, show very measurable heat at a deviation of 33°, 

 which corresponds to an index of refraction of 1*4511, while 

 the smallest index given in the plate is 1*49. 



(3) An increase of temperature increases every ordinate, 

 but not in like proportions, ordinates corresponding to the 

 heat in the more-refrangible parts always growing more 

 rapidly than those for less-refrangible heat. 



(4) As a necessary consequence of this, follows the (inde^ 

 pendently observed) fact of the progressive movement of the 

 maximum ordinate toward the more-refrangible end as the 

 temperature rises. 



(5) These prismatic curves are not symmetrical, the greater 

 portion of the area in every case lying below the maximum, 

 i. e. y toward the greater wave-length, and the descent being 

 always most abrupt on the more-refrangible side. 



As the heat-spectra from surfaces at the temperature of 



* The distinctive character of these radiations is also well shown hy 

 the fact that we have found that a thick film of lampblack, which is 

 nearly as impervious to the dark solar heat as to light, transmits more 

 than 50 per cent, of some of the ravs in question. 



2F2 



