127

Of Optics.

space IkK preferve a rectilineal course until they fall upon the top ofthe atmosphere ; and thofe which fall so about K, are refracted attheir entrance into the atmosphere, and bent down in the line Km B,to the observer’s place at B: and therefore, to him, the fun will appearat L, in the direction of the ray BmK, above the horizon B GH, whenhe is really below it at I.

The angle contained between a ray of light, and a perpendicular tothe refracting furface, is called the angle of incidence ; and the angle Angle of inti-contained between the fame perpendicular, and the fame ray aster re- dmct -fraction, is called the angle of refraBion. Thus, let L B M be the Fig. 4.refracting furface of a medium (suppofe water) and ABC a. perpendi- Angk of re-cular to that furface ; let D B be a ray of light, going out of air into^^"'water at B, and therein refracted in the line B H-, the angle AB D,is the angle of incidence, of which D F is the fine; and the angleKB H is the angle of refraction, whofe fine is KI.

When the refracting medium is water, the fine of the angle of in-cidence is to the fine of the angle of refraction, as 4 to 3 ; which is con-firmed by the following experiment, taken from Doctor Smith’sOptics.

Describe the circle DAEC on a plane square board, and crofs it atright angles with the straight lines ABC, and LBM-, then, from theinterfection A, with any opening of the compastes, fet off the equal aresAD and A E, and draw the right line DFE: then, taking Fa,which is three quarters of the length F E, from the point a, draw a Iparallel to AB K, and join Kl, parallel to BM: so KI will be equalto three quarters of FE or of DF. Thisdone, fix the board uprightupon the leaden pedestal O, and stick three pins perpendicularly intothe board, at the points D, B, and I: then fet the board upright intothe vestel FU V, and fili up the vestel with water to the line L B M.

When the water has fettled, look along the line D B , fo as you mayfee the head of the pin B over the head of the pin D ; and the pin Iwill appear in the fame right line produced to G, for its head will beseen just over the head of the pin at B: which fhews that the rayIB, coming from the pin at /, is fo refracted at B , as to proceed fromthence in the line B D to the eye of the observer; the fame as it woulddo from any point G in the right line DBG, if there were no water inthe vestel: and also fhews that KI, the sine of refraction in water, isto D F, the sine of incidence in air, as 3 to 4 *.

* This is strictly true of the red rays only, for the other coloured rays are differentlyrefracted - 3 but the difference is so fmall, that it need not be considered in this place.

Hence,