Of LigHt and Colours'.

^ill be converged to a Focus at a greaterdistance from the Lens, than the Indigo

or

Incidence I D K has been already ihewn to be asSi88 to 8047, or as 20 to 31.

18. We are now to find the Quantity of the AnodesCrDK and F D K. Since each Ray will suffer ^thebine Degree of Refraction at the second Surface as at

first very nearly; let the refracted Rays F P, ET,9 R, be produced, and they will intersect each otherlr > the Point I, making I F, I E, I G severally verypearly equal to D I, and therefore the Angles I F D —F&F, and IGD — IDG ; therefore PIV — 2 FDV,bid RIV = 2 GDV. Hence PIV — RIV = PIRba 2 FDV —2 GDV j consequently, \ PIR — FDVS 'GDV = FDG.

19. The Angle PIR st discovered by measuring thelength of the Image PR, and its Distance from the*Wm ABC. This Sir Isaac Newton has done with& r eat Exactness. The refracting Angle of his Prism^as ABC — 62° 30', the Distance of the Speilrum

Feet, the Length or 10 Inches, the Breadth 2^Fhches. This subducted from the Length leaves 7*br the Length of the Image were the Sun but a Point,bid therefore subtends the Angle which the most andbast refrangible Rays P F and R G do contain with oneMother after their Emergence from the Prism.

20. But at the Distance of 18,5 Feet, the Length

Inches is the Chord of an Arch equal to 2° 0' 7" —

JR ; therefore k PIR — FDG — i° 0 3V; whence*jE>G (=i FDG) = o° 30'2" = FDE. But the^gle EDK — DBR — 31° if o". Wherefore+ EDG = 31° 45' 2" = GDK, the Angle‘ Refraction of the least refrangible Rays ; and EDKJ^DF 30 0 44^ — EDK, the Angle of Re-

action of the most refrangible Rays.q, 2 !. The natural Sine of 31 45' 2'' is 5262, (as perFable, Annot. XLVI.) also the Sine of 30° 44^ 58 ' isati2'. The common Sine of Incidence being IDKr S D L = 53 0 35', and Sine 8047 ; this compared

witR

319