the Aberration of the fixed Stars , &c. 63 
through M muft be parallel to DB ; but DB, as before, de- 
notes the pofition of Dr. Bradley’s telefcope, when the aber- 
ration of the ftar is at its maximum, and EMO parallel to it, 
denotes the pofition of the water telefcope, at the fame time, 
upon the fuppofition that the velocity of the rays without and 
within be as EF to DF, or inverfely, as the fines of incidence 
and refraction peculiar to w T ater. Here then we difcover what 
muft be the law of variation as to the velocity of the rays, 
provided that the aberration given by fuch a telefcope (hall 
come out the fame with that found by Dr. Bradley. It is the 
very lame which follows from the Newtonian principles : for 
from the manner of obferving, the angle of aberration is 
always determined by the pofition of the telefcope neccflary 
for having the image formed fomewhere in the axis. 
But fuppofing that in the courfe of obferving with fuch a 
telefcope, the aberration fhould come out different from what 
has already been afcertained by Dr. Bradley, it may next be 
enquired, how from the difference given the velocity of light 
within the telefcope is to be deduced. 
Fig. 3. Imagine then fuch a telefcope adlually to give FMD 
as the greateft angle of aberration, and let this be fuppofed 
greater than that of Dr. Bradley’s, which, for example, let 
be FME. From what has been already faid, the velocity of 
light correfponding to this laft mentioned angle, is deducible 
from the known refraftion of the medium which fills the tele- 
fcope;. and, by conftruction, the velocity correfponding to 
FMD, the angle given, muft be to the former inverfely as the 
tangents of thefe angles. From this confideration we have the 
following analogy for finding the velocity correfponding to 
whatever difference there may be obferved between the two 
aberrations at prefent alluded to. The rule in all cafes muft be ; 
as 
