Opticks : or, A treatise of the reflections, refractions, inflections and colours of light . Light is alternately reflected and tranfmittedby thin Plates of Glafs for many Succeflions,accordingly as the thicknefs of the Plate increa-fes in an arithmetical Progreflion. For herethe thicknefs of the Glafs determines whetherthat Power by which Glafs ad:s upon Lightihall caufe it to be rcfle(5ted, or fufier it to be tranf- BOOK 11. 245 tranfmitted. And, Thirdly, becaufe thofe Sur-faces of tranfparent Bodies which have the great-eft refradling power, refled the greateft quan-tity of Light, as was fl
Opticks : or, A treatise of the reflections, refractions, inflections and colours of light . Light is alternately reflected and tranfmittedby thin Plates of Glafs for many Succeflions,accordingly as the thicknefs of the Plate increa-fes in an arithmetical Progreflion. For herethe thicknefs of the Glafs determines whetherthat Power by which Glafs ad:s upon Lightihall caufe it to be rcfle(5ted, or fufier it to be tranf- BOOK 11. 245 tranfmitted. And, Thirdly, becaufe thofe Sur-faces of tranfparent Bodies which have the great-eft refradling power, refled the greateft quan-tity of Light, as was fliewn in the firll Propofi-tion. Prop. X. If Light be fwifter in Bodies than in Vacuo, inthe proportion of the Sines which 7ncafure theRefraElion of the Bodies, the Forres of theBodies to refcB and refrafl Light, are verynearly proportional to the denfities of the fameBodies; excepting that unBuous a?id fulphureousBodies refraSt more than others of this fame den- LE T AB reprefent the refrading plane Sur-face of any Body, and IC a Ray incidentvery obliquely upon the Body in C, fo that the. Angle ACI may be Infinitely little, and let CRbe the refracfled Ray. From a given Point Bperpendicular to the refradling Surface ered: B Rmeeting with the refrading Ray C R in R, andif C R reprefent the Motion of the refradedRay, and this Motion be diftinguifhd into twoMotions CB and BR, whereof C B is paral- R 3 lei 246 O P T I C K S. lei to the refracting Plane, and BR perpendi-cular, to it: CB fhall reprefent the Motion^ incident Ray, and B R the Motion genera-ted by the Reiradtion, as Opticians have of lateexplaind. Now if any Body or Thing, in moving throughany Space of a given breadth terminated onboth fides by two parallel Planes, be urged for-ward in all parts of that Space by Forces tend-ing direftly forwards towards the laft Plane, andbefore its Incidence on the firil Plane, had noMotion towards it, -or but an infinitely littleone J and if tlie Forces in all parts of that Space
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