E. B. Hunt on the Dispersion of Light. 367 



4. No seemingly satisfactory explanation of dispersion under 

 the undulatory theory has yet been given, which is not based on 

 the hypothesis of finite intervals. 



5. This hypothesis and all others now maintained, except that 

 of the heterogeneous emission, are successful in explaining disper- 

 sion, only as they involve different velocities of propagation for 

 the different colors. 



6. If this difference of velocity be incompatible with natural 

 facts, no theory involving it can stand without modification. 



II. To show that luminous rays of different colors traverse the 

 same medium with velocities sensibly equal, we need only to 

 admit the contrary and trace its results. Suppose the red and 

 violet rays to have unequal velocities of propagation V r and V v . 

 Whatever else a ray of light may be, it is the trajectory of a se- 

 ries of impulses, which produce vision, and which take time in 

 their passage from the radiant to the eye. Two ray impulses 

 with unequal velocities V r and V v , simultaneously leaving the 

 radiant, will traverse a space, S, in the same medium in unequal 



S S 



times, T r = and T v =^. If both the radiant and the eye are 



fixed, the eye simultaneously receives two luminous impulses, 

 which left the radiant at two previous instants separated by a 

 time = T v — T r = T dif . A ray being a continuous series of these 

 impulses perfectly alike, the eye, after both red and violet have 

 reached it, would be acted on just as if T dif did not exist, and 

 would perceive this fact only by receiving R r through the space 

 T d| f before receiving R v . 



Let the eye receive a rapid motion, the radiant being still fixed. 

 This motion combined with that of the impulses along R r and 

 R v , will produce two resultants inclined to R r and R v , or astro- 

 nomical aberration will result, with two values. The compo- 

 nents of the earth's motion both along and perpendicular to these 

 rays wilt be common, while the luminous motions along them are 



unequal. Calling V e the earth's normal velocity, V e : V r =±= the 



radial component of the earth's velocity, : : 1 : tang, aberration of 

 Rr. While the tang, may be replaced by the arc. the difference 



of aberration of R r and R v = — -~ — . When V r and Vv are 



ye 



sensibly unequal, two apparent radiants will result one red and 



V r — V v 

 one violet, whose angular distance equals — ^ — . A star emit- 

 ting white light should then show as a colored spectrum with 

 this elongation. Or as the actual maximum aberration is 20"*5, 



we have XlilZJ : f> - Vv : : 20-" 5 : colored elongation. This 

 spectral elongation would be distinctly visible if it exceeded the 



