Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light. Also two treatises of the species and magnitude of curvilinear figures / By Sir Isaac Newton, Knt.

Isaac Newton

Date:: 1721

Credit: Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light. Also two treatises of the species and magnitude of curvilinear figures / By Sir Isaac Newton, Knt. Source: Wellcome Collection.

22/424 page 10

$[ reg - And by the fame means the Focus of the Rays after two or more Reflexions or Refrac- tions may be found. | — Caf4, Let ACBD [in Pg. 7.] be any refrac- — ting Lens, fpherically Convex or Concave or Plane on either fide, and let CD be its Axis (that is the Line which cuts both its Surfaces perpendicularly, and pafies through the Centers of the Spheres, ) and in this Axis produced let F and fbe the Foci of the refra¢ted Rays found as above, when the incident Rays on both fides the Lens are Parallel to the fame Axis; and upon the Diameter F f bifected in E., defcribe a Circle. Suppofe now that any Point Q be the Focus’ of any incident Rays. Draw QE. cutting the faid Circle in T and ¢, and therein take ¢ g in fuch Proportion to ¢ Kas ¢E or TE hath toT Q. Let ¢ q lye the contrary way from ¢ which TQ doth from ‘T’, and g fhall be the Focus of the refracted Rays without any fenfible Error, pro- vided the Point Q be not fo remote from the Axis, nor the Lens fo broad as to make any of the Rays fall too obliquely on the refracting Surfaces. | . And by the like Operations may the reflecting — or refracting Surfaces be found when the two Foci are given, and thereby a Lens be formed, which fhall make the Rays flow towards or from what place you pleafe. - So then the meaning of this Axiom 1s, that if Rays fall upon any Plane or Spherical Surface or Lens, and before their Incidence flow from or towards any Point Q, they fhall after Re- flexion or Refraction flow from or sipealt the jerevt oint$