La lumière ; un cours d'optique expérimentale, principalement avec la lanterne . t de l'angle de réfraction, et le twolines porte une certaine proportion ; dans le cas d'air ici andwater supposé, c'est presque exactement comme 4:3. Nowtake tout autre angle d'incidence, e c, et son rayon réfracté rayj ^ Un liquide plus lourd peut avoir moins d'^er povs de réfraction, la densité optique, ou un briquet han:un. Huile de térébenthine flotte sur l'eau, mais a le pouvoir de réfraction mutchmore. E 252 LUMIÈRE. [Chap. ce, et le dessin comme sines avant, ils porteront preciselythe satne proportion. Tous les sines portent le même invariableratio. 30.

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Light; a course of experimental optics, chiefly with the lantern . t of the angle of refraction, and the twolines will bear a certain proportion ; in the case of air andwater here supposed, it is almost precisely as 4:3. Nowtake any other angle of incidence, e c, and its refracted rayj ^ A heavier fluid may have less refractive povs^er, or optical density, :han a lighter one. Oil of turpentine floats on water, but has muchmore refractive power. E 2 52 LIGHT. [chap. ce, and drawing the sines as before, they will bear preciselythe satne proportion. All the sines bear the same invariableratio. 30. Index of Refraction.—When this ratio of the sinesis put into the form of a fraction—generally a decimal fraction—it is called the index of refraction. Unless otherwisespecified, figures so given are understood with reference toair as unity. In the case of air and water we have seen thatthis ratio s nearly^; and when put into decimals, 1-335is the index of refraction for water. It follows, that thegreater the refractive power the higher the index must be.. Fig. 35.—The Law of Sines. 31. Refraction into a Rarer Medium.—We haveproved that a ray passing obliquely from air into water isbent towards the perpendicular, or downwards; and yetif we look at a stick standing in clear water it appears tobe bent upwards. Fig. 36 explains this. The dottedlines represent the real position of the bottom part ofthe stick, and those dotted from the lowest point showthe course of the rays which reach the eye from thatpoint. On reaching the surface they are bent from theperpendicular, and the bottom of the stick is seen inthe direction from which the rays actually enter the eye. III.] TOTAL REFLECTION. 53 We thus see that the course of the refracted ray, like that ofa reflected ray, is exactly reversible. If the bottom of ourtank is made of glass, and it is raised up from the table anda ray sent up through the water, it can be shown experi-mentally that at sufficient angles the ray is r

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