the Aberration of the fixed Stars, &c. 61 
know that the ray GBF pafles into the medium without being 
refracted by it, and BMF would be confidered as the axis of 
the telefcope. 
New let the fpherical refrafting furface with its wires, cr 
the unufual telefcope be carried laterally with the motion of 
the earth towards Conceive GBF to be a line not par- 
taking of this lateral motion, which at any particular mo- 
ment paiTes thro’ M, the center of convexity. Along tjiks 
line fuppofe one of many rays to pafs from a lfar fituated in 
the pole of the ecliptic. Then will all the contemporary light 
of this pencil of parallel rays be made to converge fo as to 
meet in a focus fomewhere in the unrefra&ed ray BF. Let F 
therefore be the point in abfolute fpace where the image of the 
ftar is fo formed. Let the parallel motion of the telefcope, 
whofe refracting fpherical furface is ABC, be in the direction- 
of HF, and take FD to FB as the lateral velocity of the tele- 
fcope to the velocity of light in air, and join BD : then it is 
man i felt, that BD will be the pofition of a telefcope fuch as 
Dr. Bradley’s, when the image of the ftar is formed in the 
axis BD, and that 1 BG, or its equal FED, will be the angle 
of greateft aberration. 
Moreover,, the velocity of the rays as they proceed to the 
focus F, after refraction at the furface ABC, being iuppofed 
the fame as in air, it is evident, that the line DML drawn 
through D, and through the center of convexity M, mud give 
the pofition of the axis of this kind of telefcope, when the 
image of the ftar is formed there;, for, by hypothefis, the’ 
image is formed in F in abfolute fpace, and iince BF is iup- 
pofed to be to FD, as the velocity of light within the medium 
to the lateral velocity of the telefcope, the point D of the 
axis DL will arrive at F, when the rays arrive there to form 
the 
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