tf'2 Mr. wilson’s propofed Experiment by 
the image. And the obferver not knowing, or at prefent not 
taking account of, the lateral motion of the telelcope, will 
fuppofe, that the line LMD joining the image of the dar and 
the center of convexity M is the true direction ot the dar ; 
iuft as before he concluded, that FMBG would be the direction 
of the liar when the lateral motion of the telelcope was' fup- 
pofed to be nothing. Hence it is evident, that the interfe&ion 
of the crofs wires, made ufe of in obferving, mud; now be 
placed at D ; or elfe, if thofe be dill ufed that were before 
fuppofed to be at F, the refracting furface ABC with the line 
or axis BF mud revolve about the center M till the vertex B 
comes to L and the crols wires F to D. 
In like maimer, if the velocity of the rays were increafed 
after refraction at the fpherical furface in any ratio, as that of 
DF to EF, the refraftion continuing the fame, then EMO 
drawn through the center of convexity would now give the 
pofitioil of the axis of the telefcope neceflary for receiving the 
image formed at F. For the fpace defcribed by the rays in 
palling downwards to the focus, in this cafe and the former 
being equal, the times of their converging at F will be reci- 
procally as the velocities, or as EF to DF. But, on account 
of the equable lateral motion of the telefcope, DF and EF 
will be as the times of the points D and E arriving at F ; 
therefore, in the lad cafe, the interfeClion of the crofs wires 
fuppofed at E will meet the image at F, and accordingly the 
dar will be feen in the axis. 
Fig. 2. From what has been faid it will appear, that if DF' 
be taken to EF, as the fine of incidence to the line of refrac- 
tion peculiar to the medium which fills the telefcope ; then, 
from the property of the focus, we (hall have this proportion, 
viz. BF' •: FM :: DF' : EF. Hence the line EMO pafling 
through 
