An Explication of the Second Plate.

Figure 1. Shews that an Object, as K, seen through a plain Glass, whose Sides AB, CD, are parallel, by the Eye at G, appears out of its true Place; and this so much the more as the Glass is thicker: While at the same time the two Surfaces do exactly balance each other's Refraction, and make the two Rays HK, GF exactly parallel.

Fig.2. Exhibits a plain Method of measuring the Refraction of Fluids at all Angles, and of proving thereby that it is always in one fixed Proportion of the Sines, as the next Figure will explain it. For if the moveable Rule KCL, with its measuring Circle ABDE fix'd by the Prop E, to a heavy Pedestal FG, in a large Glass AHID, be so far immers'd in the Fluid, that the Center C may be in the Surface of the Fluid, and one of its Legs CL be so far bent from a rectilinear Position, that the Refraction of the Fluid can just make it appear as if it were in a strait Line, the Angle BCK, or its equal MCE, is the Angle of Incidence: And LCE the Angle of Refraction: And LCM the Difference, or the refracted Angle.

Fig.3. Is for the Illustration of the former Proposition, and shews the Sines afore-mentioned; as AD or GN (for they are suppos'd equal, and the Line ACN one strait Line,) is the Sine of the Angle of Incidence, and FE the Sine of the Angle of Refraction, which Sines do in the same Fluid at all Angles bear one and the same Proportion to each other; till at last, if the Refraction be out of a thick Medium into a thin one, and makes the second Sine equal to the Radius, that Ray cannot emerge at all, but will be reflected back by the Surface into the same Medium whence it came, along the Line CR.

Fig.4. Is a Bason of Water, or other Fluid; to shew the common Experiment of Refraction; where a Shilling, or other Object at A, (which is so plac'd that it cannot be seen by the Eye at O, the Side of the Bason C interposing) is readily seen there, as soon as the Water or other Fluid is put in to the same Bason, and appears to be remov'd to the Point B.

Fig.5. Is the Alteration of a round white Object D, as seen through a Triangular Glass Prism ABC, by the Eye at G, where the double Refraction of the Glass at E and F makes the Object appear at d; and that as an oblong colour'd Image; wherein the upper Part is made by the violet Rays, which are most refrangible; and the lower by the red Rays, which are least so; and the intermediate Parts by those that are refrangible in a mean Degree; after the Order of the Colours of the Rainbow.

Fig.6. Shews the Nature of a multiplying Glass AD, and its Plains AB, BC, CD, &c. and the Reason why the different Refraction of every oblique Plain, as AB, CD, &c. exhibits the same Object K as a different Object k, k, &c. according to the Number of the oblique Plains: While the direct Plain BC shews it still in its own Place: And while the Convolution of the Glass on the Axis KL removes all the oblique Images, but does not remove the direct one, on Account of the Change of the Position of those oblique Plains, and of the unchanged Position of the direct Plain.

Fig.7. Shews the Effect of the Lens, or double Convex Glass, in gathering parallel Rays, as GL, HM, AB, IN, KO, &c. towards a Point, as D; because, as in the Case of the Prism above, the Refraction to the perpendicular in the Entrance, and from it in the Exit of those Rays, do still, by the different Position of that Perpendicular, conspire to unite the same Rays.

Fig.8. Shews the contrary Effect of the double Concave Glass, in scattering the parallel Rays; and that exactly on the like Account; and so this needs no new Explication.

Fig.9. Shews the Reason why a Lens, or double Convex, shews a near Object at Q, as more remote at q, because it refracts it so that the Rays from the same Point meet more backward than before: And why it shews the same Object larger also: Which must needs be, because every Point in the Object appearing so much more backward, and yet in the same apparent Angle, its Length and Breadth must every where be proportionably enlarg'd.

Fig.10. Shews how such a Lens inverts Objects, as A, B, ba, which it does on Account of the Intersection of the Rays from each Point, in or near the Lens it self: Which necessarily infers such an Alteration: just as the Images of all Objects are in the Eye in an inverted Position, on the like Account.

Fig.11. Shews how a Lens does so refract the Rays from every Point of an Object, that is in its Focus C, and B, and A, that the Rays from each of those Points do become parallel afterward; and also how parallel Rays of different Positions are gather'd in that Focus.

Fig.12. Is the Nature of direct Vision by the Eye, in some Conformity to the 10th Figure: only in this Case the Crystalline Humour is the Lens.

Fig.13. Is the Case of a Concavo-convex Glass, with its parallel Surfaces, as in Fig.1.