4 DeWitt B. Brace, 



ing tenuity existing in interstellar spaces and closing our view- 

 more or less effectually from the infinite expanses beyond,' 

 could possibly explain these appearances, — a supposition 

 which is wholly unallowable. If there be absorption in 

 space, it must be determined by other methods than the one 

 by which Struve attempted to prove it. 



If the law of the Dissipation of Energy is absolutely uni- 

 versal, then it must be allowed that no distortion of the ether 

 can take place without a certain loss of energy however 

 small, so that the luminiferous vibrations would be gradually 

 frittered down, and after an almost infinite number of such 

 distortions be dissipated away so as to escape perception. 

 On this hypothesis, from analogy with all known phenomena 

 connected with ponderable bodies under similar conditions, a 

 differential effect should be produced for different periods of 

 vibrations, which would give a perceptible coloration in dis- 

 tant stars. 



If an excessively diffused material substance be supposed 

 scattered through space in a gaseous state, such a body could 

 only absorb selectively through its atoms, its molecules being 

 too widely scattered to allow of any transformation of energy 

 into molecular friction. Hence the only loss, other than by 

 selective absorption, would be in the ether itself. The ab- 

 sorption would then take place according to the same laws 

 which determine it when such a substance is not present. 



In a medium in which there were dissipative forces propor- 

 tional to the rate of distortion, there would be a relative 

 change in the velo(?ity of propagation of transverse vibrations 

 of different periods which, for sufficiently great distances, 

 might be detected in the coloration produced by any sudden 

 outburst or extinction of starlight. If the absorption were 

 small, such a difference in the velocities of different rays 

 would be exceedingly small, even for distances comparable 

 with the greatest dimensions of the stellar system, so that the 

 coloration could only last for a very short time. 



The luminiferous medium bears a close analogy to the 

 ponderable substances of nature in respect to its rigidity for 



4 



