64 Mr. Wilson’s propofed Experiment by 
64 as the tangent of the obferved angle is to the tangent cf the 
“ Bradleyan angle, lo is the velocity of light deducible from 
44 the hypothefis of the obferved angle being the fame with 
4 4 that of Dr. Br adley to the velocity fought.” It has already 
"been Ihewn, how the former of thefe velocities can be univerfally 
afeertained, from the known refraction of the medium which 
is taken to fill the telefcope, and therefore the lad; term of the 
above proportion, which is the velocity fought, is thereby 
given. 
Fig. 2. In a telefcope of this kind it will not have efcaped 
notice, that the ray BF, which, on account of its palling to 
the focus unrefrabted, may be called the axis of the pencil, 
can never be found in the axis of the telefcope EO, except at 
the focus F, where D and F meet. That ray, however, OP, 
parallel to BG, which falls obliquely on the axis of the tele- 
fcope EO, will continue to pafs along it after refrabtion, and 
for that reafon it may be called the relative axis of the pencil. 
This will appear, bv confidering that the particle of light, 
which at any moment is refrabted at the vertex O of the fpheri- 
cal fur face, is found by hypothef s in the axis a fecond time, 
when it meets the cotemporary light at the focus. But fincc 
both the motion of the axis and of the particle is uniform and 
rectilinear, the former cannot be found in the latter at two 
different times, without being found in it continually during 
the whole interval, hi like manner, a part of every other ray 
from the ftar, which fuceeflively falls upon the vertex, mud: 
move relatively along the axis after refraction : and thus a con- 
stant fucceffion of thefe particles conffitute a vifual refrabted 
ray, wdiofe relative path mud always be in the axis OE. 
All that has been (hewn concerning the telefcope already 
copfidered, will receive dill further illudration, by tracing the 
motion 
