Of Light? and Colours. 305

of Rays have each a peculiar Colour, viz.those which are least refrangible are red;

the

&nd M E ; upon the Point K describe the SemicirclePlQj then isNKB (= K D B) =r A K I, the Angleof Incidence out of the Prism into Air, and F K I isthe Angle of Refraction ; consequently, A R and F Sare the Sines of the Angles of Incidence and Refrac-tion out of the Prism into Air.

x 6. On the contrary, we may consider F K as theincident Ray falling upon the Prism in the Point K, andrefracted in the Direction K M parallel to the Side G H,tvhich at the Point M emerges again into the Air in thedirection ME, making the Angle EML with thePerpendicular M L, equal to the Angle F K I. In thisCafe the Angle F KI is the Angle of Incidence, andPs K B is the Angle of Refraction in the Prism ; whichAngle of Refraction is therefore given, or constant, asit is always equal to the Angle KDB, or half theAngle of the Prism.

17. The Angle of Incidence F K I consists of twoParts, viz. of the giyen Angle AKI (=; K D B) andthe additional Angle A K F. Now the Angle AKI,s known, as being equal to half the Angle of thePrism ; and the Angle F K A is known by placing thePrism by the' Centre of a graduated Semicircle, as ABC,Carrying an Index, whose two Arms F K and K E areCqually elevated above the horizontal Line AC,'andCorrespond to the incident and emergent Ray F K andAdi E in the other Figure. For here his evident, if anPbject be placed on the End of the Arm F, it will beky an E ) ,e looking through the Sights at the otherEnd of the Index E ; and when the Object is thust? e n, the Angle AKF is known by the Number ofELgrees which each Arm cuts upon the Limb of the“cmicircle.

^ 18. This Number of Degrees, added to the constantLumber 30°, which is equal to half the Angle of thetisin, gives the whole Angle of Incidence FKI; and*"Us the Angles of Incidence and Refraction being

Voi. II,

found,

PlateXXXVI.Fig. 6.