3 io THE POPULAR SCIENCE MONTHLY. 



upon the absolute or the relative velocity of light ; are the time of 

 oscillation of the particles of ether and the normal wave-length, cor- 

 responding to it, changed by any motion of translation in the origin ; 

 or is the conservation of these elements an essential attribute of the 

 luminiferous medium ? It has been said that Doppler reasoned as if 

 the corpuscular theory of light were true, and then expressed himself 

 in the language of undulations. Evidently there is an obscurity in 

 the minds of many physicists, and an uncertainty in all, when they 

 reason upon the mechanical constitution of the ether, and the funda- 

 mental laws of light. The mathematical theory is not so clear as to 

 be able to dispense with the illumination of experiment. Within the 

 present year, Van der Willigen has published a long and well-consid- 

 ered memoir on the theoretical fallacies which vitiate the whole of 

 Huggins's argument for the motion of the stars and nebula?. His anal- 

 ysis proves that the motion of the luminary will not interfere with 

 the time of oscillation and the wave-length, provided that the origin 

 of the disturbance is not a mathematical point but a vibrating mole- 

 cule, and that the sphere of action of this molecule upon surrounding 

 molecules is large enough to keep them under its influence during ten 

 or a hundred vibrations, before it is withdrawn by the motion of trans- 

 lation. If this theoretical exposition of the subject should be gener- 

 ally adopted by mathematicians, the spectroscopic observations on the 

 supposed motion of the stars must receive another interjn-etation. On 

 the other hand, if a luminary is selected which is known to move, in- 

 dependently of spectroscopic observations, and the displacement of 

 the spectrum-lines accords with this motion, it will be time to recon- 

 sider the mathematical theory, and make our conceptions of the ether 

 conform to the experiment. The spectroscopic observation of Ang- 

 strom on an oblique electric spark does not favor Huggins's views. 

 Secchi testifies to opposite displacements when he examined, with a 

 direct-vision spectroscope, the two edges of the sun's equator, one of 

 which was rotating toward him and the other from him, and Vogel 

 has repeated the observation with a reversion spectroscope. This 

 would have the force of a crucial experiment, were it not that an 

 equal displacement was seen on other parallels of latitude, and that 

 the bright bands of the chromosphere were moved, but not the dark 

 lines of the solar atmosphere. 



When Voltaire visited England in 1727, he saw at the universities 

 the effect of Newton's revolutionary ideas in astronomy. The mechan- 

 ism of gravitation had exiled the fanciful vortices of Descartes, which 

 were still circulating on the Continent. So he wrote : "A Frenchman 

 who comes to London finds many changes in philosophy as in other 

 things: he left the world full, he finds it empty." The same compari- 

 son might be made now, not so much between nationalities as between 

 successive stages of scientific development. At the beginning of this 

 century the universe was as empty as an exhausted receiver : now it 



