58 ESSAYS anp OBSERVATIONS. 
of light, z. ¢. as 1 to 77 nearly, (No, 39). 
cannot much exceed one fourth part of a 
fecond ; for the greateft aberration is but 
about twenty feconds. 
49. | HE time which the extreme violet 
takes to move through any fpace muft 
be, tothat which the red takes, as 78 to 
77. If Jupiter be fuppofed in a quadrate 
afpect with the fun, in which cafe the e- 
clipfes of his fatellites are moft commodi- 
oufly obferved, his diftance from the earth 
being nearly equal to his diftance from 
the fun; light takes about forty one mi- 
nutes of time in pafling from him to the 
earth: Therefore the laft violet light which 
a fatellite refle@s, before its total emerfion 
into the fhadow of Jupiter, ought to con-~ 
tinue to affect the eye for a 77th part of 
AY, or 32", after the red reflected at the 
| fame time is gone: That is, a fatellite, 
{een from the earth, ought to change its 
colour, above halfa minute before its total 
immerfion, from white to a livid greenith 
colour, thence into blue, and at lait eva- 
nifh in violet. I need fcarcely obferve, 
that the faid phenomznm fhould take 
place 
