the Aberration of the fixed Stars , See. 65 
motion of this particular refraded ray till it arrives at the focus. 
This way of viewing the fnbjed will alfo render the reafoning 
more general, and make it apply to telefcopes when the denfe 
fluid within is fuppofed to be confined by objed-glafles of any 
figure. But in order to this, it will be convenient to premife, 
and briefly to demonftrate, what fhall afterwards be referred to 
by the name of 
PROP. A. 
Fig. 4. If any very fmall body or particle of light as if 
moves uniformly in the abfolute path SB, has pafled relatively 
along a part of the line CD, which advances equably and 
parallel to itfelf in the diredion DK ; and if at any inftant the 
abfolute path of the particle be changed into any other, as 
BR ; I fay, it will ftill pafs relatively along the moving line, 
provided its velocity now be to its former velocity as the fine of 
the angle DBF to the fine of the angle DBR ; thefe being the 
angles which the moving line BD makes with BF and BR the 
abfolute path or diredion of the particle in the two cafes. 
The confirudion of this figure is fo fimple, that it is unne- 
ceflary formally to point it out. Since, by hypothefis, the 
velocity of the particle along BR is to its former along BF as 
the fine FZ to the fine RT ; or, on account of fimilar tri- 
angles, as DF to IR, and, on account of parallels, as DF to 
DW, it follows, that the time of its deferibing BR now, is 
to the time of formerly deferibing its equal BF, as DW to DF. 
But the line BD advancing with a uniform motion, the time of 
its arriving at W is to the time of its arriving at F, alfo as DW to • 
DF. Therefore, when the particle arrives at R, the point D of the 
moving line will have arrived at W, and WRP will be its posi- 
tion. Hence the particle at that moment mud be found in the 
interfedion R of this line, with its abfolute path BR. In the 
Vol. LXXII. K fame 
