So then the Meaning of this Axiom is, that if Rays fall upon any Plane or Spherical Surface Or Lens, and before their Incidence flow from or towards any Point 2, they shall after Reflexion or Refraction flow from or towards the Point q found by the foregoing Rules. And if 9 the incident Rays flow from or towards several points Q, the reflected or refracted Rays shall flow from or towards so many other Points q found by the same Rules. Whether the reflected and refracted Rays flow from or towards the Point q is easily known by the situation of that Point. For if that Point be on the same side of the reflecting or refracting Surface or Lens with the Point Q, and the incident Rays flow from the Point Q, the reflected flow towards the Point q and the refracted from it; and if the 9 incident Rays flow towards Q, the reflected flow from and the refracted towards it. And the contrary happens when q is on the other side of the Surface.

A X. VII.

Wherever the Rays which come from all the Points of any Object meet again in so many points after they have been made to converge by Reflection or Refraction, there they will make a PiEture of the Object upon any white Body on which they fall.

So if PR [in Fig. 3.] represent any object, without Doors, and AB be a Lens placed at a hole in the Window-shut of a dark Chamber, whereby the Rays that come from any Point Qof

that

that Object are made to converge and meet again in the Point q; and if a Sheet of white Paper be held at q for the Light there to fall upon it, the picture of that Object PR will appear upon the Paper in its proper shape and Colours. For as the Light which comes from the Point Qgoes to the Point q, so the Light which comes from other Points P and R of the Object, will go to so many other correspondent Points p and r (as is manifest by the sixth Axiom;) so that every Point of the Object shall illuminate a correspondent Point of the Picture, and thereby make a picture like the Object in Shape and Colour, this only excepted, that the picture shall be inverted. And this is the Reason of that vulgar Experiment of casting the Species of Objects from abroad upon a Wall or Sheet of white Paper in a dark Room.

In like manner, when a Man views any Object PQR, [in Fig. 8.] the Light which comes from the several Points of the Object is so refracted by the transparent skins and humours of the Eye, (that is, by the outward coat E F G, called the Tunica Cornea, and by the crystalline humour A B which is beyond the Pupil mk) as to converge and meet again in so many Points in the bottom of the Eye, and there to paint the picture of the Object upon that skin (called the Tunica Retina) with which the bottom of the Eye is covered. For Anatomists, when they have taken off from the bottom of the Eye that outward and most thick Coat called the Dura Mater, can then fee through the thinner Coats, the Pictures of Objects lively painted there

on.

on. And these Pictures, propagated by Motion along the Fibres of the Optick Nerves into the Brain, are the cause of Vision. For accordingly as these pictures are perfect or imperfect, the Object is seen perfectly or imperfectly. If the Eye be tinged with any colour (as in the Disease of the Jaundice) so as to tinge the Pictures in the bottom of the Eye with that Colour, then all objects appear tinged with the same Colour. If the Humours of the Eye by old Age decay, so as by shrinking to make the Cornea and Coat of the Crystalline Humour grow flatter than before, the Light will not be refracted enough, and for want of a sufficient Refraction will not converge to the bottom of the Eye but to some place beyond it, and by confequence paint in the bottom of the Eye a confused picture, and according to the Indistinct ness of this Picture the Object will appear confused. This is the reason of the decay of sight in old Men, and shews why their Sight is mended by Spectacles. For those Convex glasses supply the defect of plumpness in the Eye, and by increasing the Refraction make the Rays converge sooner, so as to convene distinctly at the bottom of the Eye if the Class have a due degree of convexity. And the contrary happens in short-sighted Men whose Eyes are too plump. For the Refraction being now too great, the Rays converge and convene in the Eyes before they come at the bottom; and therefore the Picture made in the bottom and the Vision caused thereby will not be distinct, unless the object be brought so near the Eye as that the place

place where the converging Rays convene may be removed to the bottom, or that the plumpness of the Eye be taken off and the Refracti ons diminished by a Concave-glass of a due degree of concavity, or lastly that by Age the Eye grow flatter till it come to a due Figure: For short-sighted Men fee remote Objects best in Old Age, and therefore they are accounted to have the most lasting Eyes.

A X. VIII.

An Object seen by Reflexion or Refraction, appears in that place from whence the Rays after their last Reflexion or Refraction diverge in falling on the Spectator's Eye.

If the Object A [in Fig.9.] be seen by Reflexion of a Looking-glass m n, it shall appear, not in its proper place A, but behind the Glass at a, from whence any Rays AB, AC, AD, which flow from one and the same Point of the Object, do after their Reflexion made in the Points B, C, D, diverge in going from the Glass to E, F, G, where they are incident on the Spectator's Eyes. For these Rays do make the same Picture in the bottom of the Eyes as if they had come from the Object really placed at a without the Interposition of the Looking-glass; and all Vision is made according to the place and shape of that Picture.

In like manner the Object D [in Fig. 2.] seen through a prism, appears not in its proper place D, but is thence translated to some other place d situated in the last refracted Ray FG drawn backward from F to d.

And

And fo the Object Q[ in Fig. 10.] seen through the Lens A B, appears at the place q from whence the Rays diverge in passing from the Lens to the Eye. Now it is to be noted, that the Image of the Object at q is so much bigger or lesser than the object it self at Q, as the distance of the Image at q from the Lens A B is bigger or less than the distance of the object at Q from the same Lens. And if the object be seen through two or more such Convex or Concave-glasses, every Glass shall make a new Image, and the Object shall appear in the place of the bigness of the last Image. Which consideration unfolds the Theory of Microscopes and Telescopes. For that Theory consists in almost nothing else than the describing such Glasses as shall make the last Image of any Object as distinct and large and luminous as it can conveniently be made.

I have now given in Axioms and their Explications the sum of what hath hitherto been treated of in Opticks. For what hath been generally agreed on I content my self to assume under the notion of Principles, in order to what I have farther to write. And this may suffice for an Introduction to Readers of quick Wit and good Understanding not yet versed in Opticks: Although those who are already acquainted with this Science, and have handled Glasses, will more readily apprehend what followeth.

PROPO