( 6n ) 
THE ffloft refrangible Rays confift of fmaller Par- 
ticles than the leaft refrangible Rays, and therefore 
muft have leaft Momentum , the Velocity of all th^ 
Rays being the fame ; and confequently are more 
eafilyturned out of the Way by Attraction or Re- 
pulfion, which makes the Curves made by the pur- 
ple and violet Rays under the Surface A B, to be lefs 
and nearer the faid Surface than the Curves made by 
red and orange Rays. 
Suppole a Violet R a moving in the Direction R r 
{ Fig. 1 6 .) to be fo bent under the Surface A B, that 
at the Vertex of the Curve, or where its Tangent cc 
is parallel to A B, there (till remains a fmall Space be- 
tween the Curve and the Line n n , where the Limits 
of Attraction ( contracted by the Proximity of the 
undermoft Prifm D F G end) that Ray will be turn- 
ed up again in the Curve d e fi and fo reflected in 
the Line /f, the Directions having been fucceftively 
changed, as in Fig. 14. But a red Ray with the 
fame Inclination, would pafs on into the lower 
Prifm, as was explained in Fig. 15*. Becaufc the 
Momentum of the red Ray being greater than that of 
the Violet, the fame Degree of Attraction could not 
give it the fame Flexure. 
This is confirmed by Experiment, for when the 
lower Prifm is not preffed hard againft the upper 
(as in Fig. 11.) the Rays brought down to R make 
a Spot of a Colour made up chiefly of red and orange 
Rays ; but when the Prifms are preffed clofer, the 
Spot grows bigger and perfectly white in its Middle, 
becaufe all Sorts of Rays are brought down to the 
Spot ; but it is inclofed round with a reddiih Border, 
occafioned by the. Parts of the Prifm which are very 
near. 
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