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hypothefis, that a fatellite, feen from the earth, ought 
to change its colour about half a minute before its 
total immerfion, from white to a livid greenilh co- 
lour ; thence into blue, and at laft evanifh in violet. 
I need hardly obferve, that the fame phenomenon, 
mull take place in the time of emerfion by a con- 
trary fucceffion of colours, beginning with red, and 
ending in white. 
12. If this phenomenon be perceived by aftrono- 
mers, we fhall have a direCt proof of the different 
velocities of the differently-colour’d rays, and, con- 
fequently, a mechanical account of their different de- 
grees of refrangibility ; for I fee not, to what other 
caufe fuch an appearance could be reafonably afcribed. 
If it be not, we may conclude, that rays of all co- 
lours are emitted from the luminous body with one 
common velocity. 
Geneva, Feb. i, 1753. 
T. MelvilL 
This paper was delivered to Mr. Short, in order to 
attend to the particular obfervation of Jupiter’s 
fatellites recommended by Mr. Melvill, who 
after fome time made the following report to 
the Society. 
f 
OVER fince this paper of Mr. MelvilPs was put 
into my hands, I have carefully attended the 
emerlions of Jupiter’s firff fatellite thro’ a reflecting 
telefcope of four feet focal length, and with a proper 
magnifying power j but I have not perceived the lead; 
alteration in the colour of the light reflected by the 
fatellite, 
I 
