PRIMARY CONCEPTS OF MODERN SCIENCE. 713 



rays, which together make up white light, concur in their action on 

 the retina at a given moment; in ordinary cases it is immaterial 

 whether these rays have left the luminous body successively or together. 

 But it is otherwise when a luminous body becomes visible suddenly, 

 as in the case of the satellites of Jupiter, or Saturn, after their eclipses. 

 At certain periods, more than 49 minutes are requisite for the trans- 

 mission of light from Jupiter to the earth. Now, at the moment when 

 one of Jupiter's satellites, which has been eclipsed by that planet, 

 emerges from the shadow, the red rays, if their velocity were the great- 

 est, would evidently reach the eye first, the orange next, and so on 

 through the chromatic scale, until finally the complement of colors 

 would be filled by the arrival of the violet ray, whose velocity is 

 supposed to be the least. The satellite, immediately after its emersion, 

 would appear red, and gradually, in proportion to the arrival of the 

 other rays, pass into white. Conversely, at the beginning of the 

 eclipse, the violet rays would continue to arrive after the red and 

 other intervening rays, and the satellite, up to the moment of its total 

 disappearance, will gradually shade into violet. 



Unfortunately for Cauchy's hypothesis, the most careful observation 

 of the eclipses in question has failed to reveal any such variations of 

 color, either before immersion, or after emersion, the transition between 

 light and darkness taking place instantaneously, and without chro- 

 matic gradations. 



If it be said that these chromatic gradations escape our vision by 

 reason of the inappreciability of the differences under discussion, as- 

 tronomy points to other phenomena no less subversive of the doctrine 

 of unequal velocities in the movements of the chromatic undulations. 

 Fixed stars beyond the parallactic limit, whose light must travel more 

 than three years before it reaches us, are subject to great periodical 

 variations of splendor ; and yet these variations are unaccompanied 

 by variations of color. Again, the assumption of different velocities 

 for the different chromatic rays is discountenanced by the theory of 

 aberration. Aberration is due to the fact that, in all cases where the 

 orbit of the planet, on which the observer is stationed, forms an angle 

 with the direction of the luminar ray, a composition takes place be- 

 tween the motion of the light and the motion of the planet, so that 

 the direction in which the light meets the eye is a resultant of the two 

 component directions the direction of the ray and that of the ob- 

 server's motion. If the several rays of color moved with different 

 velocities there would evidently be several resultants, and each star 

 would appear as a colored spectrum longitudinally parallel to the 

 direction of the earth's motion. 



The alleged dependence of the velocity of the undulatory move- 

 ments, which correspond to, or produce, the different colors, upon the 

 length of the waves, is thus at variance with observed fact. The 

 hypothesis of definite intervals is unavailable as a supplement to the 



