[ i8 ° ] 
Thefe numbers are partly tranfcribed from Sir Ifaac 
Newton, and partly computed by a rule of Mr. Eu- 
ler in the Philafophical Tranfadtions. 
They are indeed carried on to more decimal places 
than the experiments hitherto made can well bear: 
but it is hoped that hereafter methods may be devif- 
ed to meafure the refractions of light to a very great 
degree of preciiion. 
9. When a {lender pencil SO, is refraCted by the 
furface of a denfer medium OT (Fig. 3.) the 
extreme rays being OV, the violet, and OR the red; 
we have feen that the furface RVT, at which the 
rays pafs again into the rarer medium, being parallel 
to the fir ft furface OT, the extreme, and all the in- 
termediate, rays will emerge parallel to each other, 
and to the pencil SO. 
But if the laft furface RVT cuts the former in a 
line perpendicular to the plane of refradtion at the 
point T, on the fide of the radiant point S, then the 
extreme rays being refraCted at the points V, R, will 
converge to fome point F in the rarer medium : and 
if the light be received on a fcreen at F, it will be 
colourlefsj if nearer to the refra&ing medium, or 
farther from it, it will be tinged, but on different 
fides. 
Thus if the denfer medium is water, and the fur- 
rounding medium is air; the angle of incidence LOS 
being 20°, the angle of divergence V O R will be 
7' 46". And O VP the angle of incidence at the 
fecond refradtion for the violet rays being taken of 
30°, the angle of convergence RFY will be 14' 26". 
On the contrary, if the plane VR/, (Fig. 4.) which 
terminates the denfer medium cuts the firit refradting 
plane 
