OPTICKS.
An Explication of the First Plate.
Figure 1. Represents the Foundation of Vision, and of all Opticks whatsoever, by exhibiting to the Eye a Specimen how the Rays of Light do as well originally, as after Reflection or Refraction, spread themselves in right Lines from each Point in every visible Object, as P, to each other Point, as R, R, R, R, R, every way, to be receiv'd by the Eye in any direct Position whatsoever.
Fig. 2. Represents the known Law of Reflection; that the Angle of Incidence C P D, is equal to that of Reflection C P E, or that the Angle of Inclination D P A is equal to the other E P B.
Fig. 3. Shews the Reason why a plain Looking-Glass, as A E F B, exhibits the Object C D by the Image c d, which is equal to C D, and equidistant from the Glass A c = A C: And in an erect Posture; all depending only on the Equality of the Triangles, whose Vertices are C c : D d, and have their common Bases below E and above F, which Glass by forming the same Image c d, so to the Eye, as if the real Object C D was at c d, must needs shew that Picture in the Place assign'd, without any Inequality of Distance or Magnitude, or any Inversion.
Fig. 4. Shews the Reason why the same or equal Object, as A B, C D, E F, appears larger when it is nearer, and smaller when farther off: viz. on account of the Inequality of the Angles A G B, or M G N, and C G D, or K G L, and E G F or H G I, and the consequent Inequality of the Pictures made by the Rays at the Bottom of the Eye.
Fig. 5. Shews the Reason why a Convex Looking-Glass, as A E F B, exhibits Object C D by the Image c d, both nearer to the Glass, and lesser than it self; but still in an erect Posture. All depending only on the different Bend of the Circle between E and its lower Point, between F and its upper Point; which cannot make the Angles of Reflection or Inclination equal, as they must needs be in all such Reflections, without making the Vertices of the Angles, as c and d, nearer the Glass than C and D: And so the apparent Picture or Diameter c d lesser than that of the Object C D, though without any Inversion.
Fig. 6. Shews the Reason why a Concave Glass, as A E F B, exhibits an Object plac'd nearer the Glass than the Center, as C D by the Image c d, remoter from the Glass, and larger than it self, viz. for Reasons just contrary to those under the fifth Figure foregoing.
Fig. 7. Shews the Reason why a Concave Glass, as C D E F, exhibits an Object, if it be plac'd remoter than the Center, as A B, inverted, and at different Distances between the Eye and the Glass; according to the Length or Shortness of its own Distance, as B C or A D, viz. Because the Rays from the same Point still cross one another, as at G and H, before they fall upon the Eye; and so by forming an inverted Image make it impossible for the Eye to see the Object in any other Position than that the Image has; which Image indeed it self is the only proper Object of the Eye, in all such Cases whatsoever.
Fig. 8. Is a Picture in Confusion; but rectified by a Convex Cylinder, and thereby brought into exact Order again.
Fig. 9. Represents an Image in a Cylindrical Concave Surface, when the Eye is in a Plain perpendicular to its Axis; so that lengthways it is as a Plain, and breadthways as a Concave Speculum: Which therefore makes the Picture longer, but not wider. The contrary will happen in a Convex Speculum, which will make it shorter but not narrower, for the like Reason.
