400 Intelligence and Miscellaneous Articles. 



missibility through the trough of spar filled with water was remark- 

 ably increased ; it became £| in the first case, and Jj- in the other. 



From these numbers a complete demonstration of the propositions 

 previously enunciated appears to follow. In fact, if the trough filled 

 with water only allows a third of the rays near the extreme red to 

 pass, and far less of the other and less refrangible obscure pencils, 

 it is impossible to assume that a mixture of rays so little transmis- 

 sible could form a pencil capable of being transmitted in the propor- 

 tion of i--| or -|. Yet this consequence must be admitted if it be 

 allowed that the trough of water only modifies the incident pencils 

 in the degree that it clears them by extinguishing rays of mean re- 

 frangibility very different from those of the pencils investigated. 



It seems to me made out that if in very fine spectra, but of differ- 

 ent sources, we isolate two layers of rays of the same mean refran- 

 gibility, and the breadth of which (the same in both cases) is a very 

 small portion of the total breadth of the spectrum, these two layers 

 of rays may have very different properties. 



Without discussing here the various interpretations which might 

 be proposed of this fact, I will restrict myself to the remark that it 

 may be completely explained by the data which spectrum-analysis 

 furnishes. In fact, if in a solar spectrum, whether luminous or che- 

 mical, we consider a band the breadth of which is a fifteenth to a 

 thirtieth of that of the spectrum, we know that this band is filled with 

 a number of lines which correspond to rays that have been absorbed 

 in passing through the solar or terrestrial atmosphere ; and, though 

 with considerable differences, absorbents conveniently chosen may 

 produce analogous effects on spectra from terrestrial sources. Now 

 the lines (the bands of absorption in question) being developed in a 

 pencil of definite mean refrangibility, necessarily modify it, both as 

 regards intensity and transmissibility, since obviously the rays which 

 have disappeared were not the same as those which remained, though 

 they had the same degree of refrangibility. 



In concluding, I beg to mention a fact which is directly connected 

 with the study of calorific spectra. A body raised to a red heat 

 emits both dark and luminous calorific rays. If it be heated from 

 the point at which it becomes distinctly luminous, is the increase in 

 the energy of its radiation restricted to the luminous part alone, or does 

 it extend to both at once ? The simple fact that the maximum heat is 

 greatest in the dark part of the solar spectrum seems to indicate that 

 the second hypothesis is true. It may moreover be verified in the 

 following manner. We take as source of heat a plate of platinum 

 forming a side of a small gas-furnace, in which gradually higher 

 temperatures may be obtained by increasing the pressure of the air. 

 When the plate is at a distinct cherry-red, part of the rays it emits 

 are isolated and dispersed by a prism. The pile, placed in the ob- 

 cure part of the spectrum at a considerable distance from the lumi- 

 nous part, remains fixed as long as the pressure of the air injected is 

 the same ; if the pressure be increased, the plate passes from cherry 

 to white, and at the same time the thermoscopic indication is greatly 

 increased, although the pile cannot receive any luminous ray. — 

 Comptes Rendus, August 3, 1868. 



i 



