Homogeneous medium by direct or straight lines extended every way like Rays from the center of a Sphere. Fifthly, in an Homogeneous medium this motion is propagated every way with equal velocity, whence necessarily every pulse or vitration of the luminous body will generate a Sphere, which will continually increase, and grow bigger, just after the same manner (though indefinitely swifter) as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it, where, by the sinking of a Stone the motion was begun, whence it necessarily follows, that all the parts of these Spheres undulated through an Homogeneous medium cut the Rays at right angles.
But because all transparent mediums are not Homogeneous to one another, therefore we will next examine how this pulse or motion will be propagated through differingly transparent mediums. And here, according to the most acute and excellent Philosopher Des Cartes, I suppose the sign of the angle of inclination in the first medium to be to the sign of refraction in the second, As the density of the first, to the density of the second. By density, I mean not the density in respect of gravity (with which the refractions or transparency of mediums hold no proportion) but in respect onely to the trajection of the Rays of light, in which respect they only differ in this; that the one propagates the pulse more easily and weakly, the other more slowly, but more strongly. But as for the pulses themselves, they will by the refraction acquire another propriety, which we shall now endeavour to explicate.
We will suppose therefore in the first Figure A C F D to be a physical Ray, or A B C and D E F to be two Mathematical Rays, trajected from a very remote point of a luminous body through an Homogeneous transparent medium L L L, and D A, E B, F C, to be small portions of the orbicular impulses which must therefore cut the Rays at right angles; these Rays meeting with the plain surface N O of a medium that yields an easier transitus to the propagation of light, and falling obliquely on it, they will in the medium M M M be refracted towards the perpendicular of the surface. And because this medium is more easily trajected then the former by a third, therefore the point C of the orbicular pulse FC will be mov'd to H four spaces in the same time that F the other end of it is mov'd to G three spaces, therefore the whole refracted pulse GH shall be oblique to the refracted Rays C H K and G I; and the angle G H C shall be an acute, and so much the more acute by how much the greater the refraction be, then which nothing is more evident, for the sign of the inclination is to the sign[errata 1] of refraction as G F to T C the distance between the point C and the perpendicular from G on C K, which being as four to three, H C being longer then G F is longer also then T C, therefore the angle G H C is less than G T C. So that henceforth the parts of the pulses G H and I K are mov'd ascew, or cut the Rays at oblique angles.
It is not my business in this place to set down the reasons why this or that body should impede the Rays more, others less: as why Water should transmit the Rays more easily, though more weakly than air. Onely thus
Errata
