368 E. B. Hunt on the Dispersioji of Light. 



minimum observable distance. But no such appearance is observ- 

 ed ; hence a very low limit of possible values for V r - V v is fixed : 

 a value obviously too small to explain dispersion. 

 . If a rapid motion be given to the radiant, a like spectral elon- 

 gation will result from assigning unequal velocities to the differ- 

 ent colored rays. The red ray would show a position of the ra- 

 diant T' -T r units of time later than that chosen by R v simul- 

 taneously received. This spectral distortion would tend to cor- 

 rect or magnify that due to aberration, according as the earth 

 and radiant have like or opposed motions. This should cause 

 distortions of planetary discs and cometic nuclea, actually unob- 

 served. 



All instantaneous events, like eclipses and emersions of Jupi- 

 ter's satellites occurring at great distances, should if V r >V v , 

 finally disappear in violet, and first appear in red gradually grow- 

 ing white. The eclipses and emersions of Jupiter's satellites are 

 so sharply defined, that they serve well for determining longitude. 

 About 49' 23" may intervene between an event at Jupiter and 

 our perception of it. About l h 26£ may pass after an eclipse or 

 emersion of Saturn's satellites, before we see it. Yet their al- 

 most instantaneous transitions between light and darkness reveal 

 to us no long lingering violet or swiftly rushing red. 



For fixed stars situated beyond the parallactic limit, the time 

 required for propagating their light to us cannot be less than three 

 years, but may be indefinitely greater. The star Algol or (8 Persei, 

 "is usually visible as a star of the second magnitude, and such 

 it continues for the space of 2 d 14 b , when it suddenly begins to 

 diminish in splendor, and in 3£ hours is reduced to the fourth 



# 



* After maturing 



g in my own mind the previous ideas, my attention was attracted 

 tothi subjoined passage in Lloyd's Report, already mentioned. "Why its applica- 

 bility to a difference of chromatic velocities in all theories was not perceived, I do 

 not set-. 



L de Courtivon and Mr 

 difference in the initial 



violet slowest But were such the cause of the phenomenon, the dispersion should 

 be proportionate to the mean refraction. Indeed the hyp., thesis was abandoned al- 

 m< m men as proposed. Its authors had foreseen the consequence that, in the 

 eclips, of Jupiter e satellites, the color of the light should vary just before immer- 

 sion ami after emersion ; and the existence of such an effect in the degree indicate. 



I theory^ was completely disproved bv the observation! of Mr. S) rt Another 

 consequence of such a difference in the initial velocities of light of different colors 



is, that the aberration of the fixed stars should also vary with the nature of the light, 

 and each star appear as a colored spectrum, whose length is parallel to the direc- 

 tion of the earth's motion." 



f "The duration of this change, according to Mr. Melville, should amount to thirty 

 two s onds, the velocities of the light of different colors I ng ini •sely M their re- 

 fractive indices. This principle, however, a- M. Clairaut hi shown is obviously in- 

 correct. It wjU isiiy appear that the initial velocities nn t vary inversely as the 

 quantity Jp.2 _ l, m or j er to account for dispersion; and that the duration of the 

 expected phenomenon must be even greater than that assiened bv Mr. Melville." 



