enter the eye, to have their pulses made oblique to their progression, and consequently each Ray to have potentially superinduc'd two proprieties, or colours, viz., a Red on the one side, and a Blue on the other, which notwithstanding are never actually manifest, but when this or that Ray has the one or the other side of it bordering on a dark or unmov'd medium, therefore as soon as these Rays are entred into the eye and so have one side of each of them bordering on a dark part of the humours of the eye, they will each of them actually exhibit some colour; therefore A D C the production of G A C H[errata 1] will exhibit a Blue, because the side C D is adjacent to the dark medium C Q D C, but nothing of a Red, because its side A D is adjacent to the enlightned medium A D F A: And all the Rays that from the points of the luminous body are collected on the parts of the Retina between D and F shall have their Blue so much the more diluted by how much the farther these points of collection are distant from D towards F; and the Ray A F C the production of K C A I, will exhibit a Red, because the side A F is adjacent to the dark or quiet medium of the eye A P F A, but nothing of a Blue, because its side C F is adjacent to the enlightned medium C F D C, and all the Rays from the intermediate parts of the luminous body that are collected between F and D shall have their Red so much the more diluted, by how much the farther they are distant from F towards D.
Now, because by the refraction in the Cornea, and some other parts of the eye, the sides of each Ray, which before were almost parallel, are made to converge and meet in a point at the bottom of the eye, therefore that side of the pulse which preceded before these refractions, shall first touch the Retina, and the other side last. And therefore according as this or that side, or end of the pulse shall be impeded, accordingly will the impressions on the Retina be varied; therefore by the Ray G A C H refracted by the Cornea to D there shall be on that point a stroke or impression confus'd, whose weakest end, namely, that by the line C D shall precede, and the stronger, namely, that by the line A D shall follow. And by the Ray K C A I refracted to F, there shall be on that part a confus'd stroke or impression, whose strongest part, namely, that by the line C F shal precede, and whose weakest or impeded, namely, that by the line A F shall follow, and all the intermediate points between F and D will receive impressions[errata 2] from the converg'd Rays so much the more like the impressions on F and D by how much the nearer they approach that or this.
From the consideration of the proprieties of which impressions, we may collect these short definitions of Colours: That Blue is an impression on the Retina of an oblique and confus'd pulse of light, whose weakest part precedes, and whose strongest follows. And, that Red is an impression on the Retina of an oblique and confus'd pulse of light, whose strongest part precedes, and whose weakest follows.
Which proprieties, as they have been already manifested, in the Prisme and falling drops of Rain, to be the causes of the colours there generated, may be easily found to be the efficients also of the colours appearing in thin laminated transparent bodies; for the explication of which, all this has been premised.
Errata
