Mr.
Fermat was the first to propound this property of refraction, holding with us, and directly counter to the opinion of Mr.Des Cartes, that light passes more slowly through glass and water than through air.
But he assumed besides this a constant ratio of Sines, which we have just proved by these different degrees of velocity alone: or rather, what is equivalent, he assumed not only that the velocities were different but that the light took the least time possible for its passage, and thence deduced the constant ratio of the Sines.

His demonstration, which may be seen in his printed works, and in the volume of letters of Mr.Des Cartes, is very long; wherefore I give here another which is simpler and easier.
[Illustration] Let KF be the plane surface; A the point in the medium which the light traverses more easily, as the air; C the point in the other which is more difficult to penetrate, as water.

And suppose that a ray has come from A, by B, to C, having been refracted at B according to the law demonstrated a little before; that is to say that, having drawn PBQ, which cuts the plane at right angles, let the sine of the angle ABP have to the sine of the angle CBQ the same ratio as the velocity of light in the medium where A is to the velocity of light in the medium where C is.

It is to be shown that the time of passage of light along AB and BC taken together, is the shortest that can be.

Let us assume that it may have come by other lines, and, in the first place, along AF, FC, so that the point of refraction F may be further from B than the point A; and let AO be a line perpendicular to AB, and FO parallel to AB; BH perpendicular to FO, and FG to BC.
Since then the angle HBF is equal to PBA, and the angle BFG equal to QBC, it follows that the sine of the angle HBF will also have the same ratio to the sine of BFG, as the velocity of light in the medium A is to its velocity in the medium C.But these sines are the straight lines HF, BG, if we take BF as the semi-diameter of a circle.