55

Optics.

* ncl its Bulk or Solidity 64000 times.

(cxxvm)

If

tzion l ' 'F^ Nature and Structure of a com-

e vide re *s a< ^’ n g Telescope is above described, and is sor e ] a t; rit om the Figure, that I so all fay nothing farthersoe 1°^ t0 * tS Compo' tion, but soall proceed to soewfrom 1 ?/ 361 ^ 6 ^’ 011 t ^ 1 ‘ s Telescope, and that it arises

Hot sr le different Refrangibility of common Light, andd ans the spherical Figure of the Glass, as the Opti-tberes bef ° re ^ ir V aac Newton’ s Time imagined, andby • ° n ' P^pofed to bring them to greater PerfectionQ] a flT°d uc ing the Method of grinding and polishingAliens °^" t ^ e Figure of one or other of the Conic

IViy ut this great Philosopher soon soewed them theirfigure e ’- by P rov ‘ n g that the Error arising from thethat 0t t ^ e Olass was many hundred times less thanOf th e Proceeded from the unequal Refrangibilitysid era , | y s > and was so small as to be altogether incon-an d this he did by an elegant Method in hisii\rJ 0nes Optius, which I soall here translate from that^'rable fiopk.

C Z 7s et: N B M be a spherical Surface, C the Centre,kay S a Semidiameter or Axis parallel to the incidentfra(£f’| FT an incident Ray, and NK the fame re-ICj cutting the Axis CB produced in the Pointthe ft ■ F he the Solar or principal Focus, where

Axis meet the Axis which are infinitely near to the

4.. T r C F rr °r K F is now to be determined.

1^ q et ™1 the Perpendiculars C E upon N K, andG*. Upo " CK > and call C B zza, GB=*, and CK_ 1 ar, h from the Nature of the Circle we have N G 1

Tl 2 a x — xx, to which add G K 1 —-(* q? 4-2 xz —2 a z 2 x 4-a 1 , and th

5- Now since N G : CE :: I: R, » f. h , s t a e^ Incidence to the Sine of Refraction ; and decani ^similar Triangles C E K and N G K, it is N G : ^ * -

^••CKaLR; therefore T : R* == (N K*jCJM* + 2 x z — 2 a % + : a 1 ? and 1 » — ^

E-4

PlateXLVII.Fig, i-