Diftance, Magnitude, &c. of the Fixed Stars, 8c. st 
"30. The diminution of the velocity of light, in cafe it 
fhould be found to take place in any of the fixed flars, is the 
principal phenomenon whence it is propofed to difcover their 
diftance, &c. Now the means by which we may find what 
this diminution amounts to, feems to be fupplied by the dif- 
ference which would be occafioned in confequence of it, in the 
refrangibility of the light, whofe velocity fhould be fo dimi- 
minifhed. For let us fuppofe with Sir Isaac Newtown (fee 
his Optics, prop. vi. paragr. 4 and 5) that the refraction 
of light is occafioned by a certain force impelling it to- 
wards the refraéting medium, an hypothefis which perfee- 
ly accounts for all the appearances. Upon this hypothefis 
the velocity of light in any medium, in whatever direétion it 
falls upon it, will always bear a given ratio to the velocity it 
had before it fell upon it, and the fines of incidence and re- 
fraction will, in confequence of this, bear the fame ratio to 
each other with thefe velocities inverfely. Thus, according to 
this hypothefis, if the fines of the angles of incidence and 
refraction, when light pafles out of air into glafs, are in the 
ratio of 31 to 20, the velocity of light in the glafs muft be to 
its velocity in air in the fame proportion of 31 to20. But be- 
caufe the areas, reprefenting the forces generating thefe veloci- 
tics, are as the {quares of the velocities, fee art. 5. and 6. thefe 
areas muft be to each other as g61 to goo. And if 400 tepre- 
fents the area which cor refponds to the force producing the ori+ 
ginal velocity of light, 561, the difference between 961 and 
400, muft reprefent the area correfponding to the additional 
force, by which the light was ielielae at the furface of the 
glafs. 
5 31. In art. 19. we fuppofed, by way of example, the velo- 
city of the light of fome particular ftar to be diminifhed in the’ 
Hig ratio 
