Elements of natural philosophy (Volume 2-3) . If an eye be placed in a more refracting medium Appearances duothan the atmosphere, as that of a fish under water, it willtothehmitof x 7 refraction and perceive, by the limit of refraction, all objects in the total reflexion;horizon elevated in the air, and brought within 48° 15of the zenith, while some objects in the water would4 ap-pear to occupy the belt included between this limit andthe horizon by total reflexion. Those remarkable cases of mirage, where objects areseen suspended in the air, and oftentimes inverted, are Those due toexplained b
Elements of natural philosophy (Volume 2-3) . If an eye be placed in a more refracting medium Appearances duothan the atmosphere, as that of a fish under water, it willtothehmitof x 7 refraction and perceive, by the limit of refraction, all objects in the total reflexion;horizon elevated in the air, and brought within 48° 15of the zenith, while some objects in the water would4 ap-pear to occupy the belt included between this limit andthe horizon by total reflexion. Those remarkable cases of mirage, where objects areseen suspended in the air, and oftentimes inverted, are Those due toexplained by ordinary refraction and total reflexion. or1,nary -»- * * refraction and The phenomena of mirage most frequently occur when total reflexion,there intervenes between the suspended object and spec-tator a large expanse of water or wet prairie, and towardsthe close of a hot and sultry day, when the air is calm,so that the different strata may arrange themselves ac-cording to their different densities. When the windrises the phenomena -M Illustration; It is well known that in the ordinary state of the at- Apparentmosphere, its density decreases as we ascend; a ray 0f cfccto[thc 1 d atmosphere on light, therefore, entering the atmosphere at S, would uh- the positions ofdergo a series of refractions, and reach the eye at .#, with ce,estial b0(lio^an increased inclination to the surface of the earth ; andwould appear to come from a point, S\ in the heavensabove that at 6) occupied by a body from which it pro- 182 NATURAL PHILOSOPHY. ceeded. Hence, the effect of the atmosphere is to in-crease apparently the altitudes of all the heavenly-bodies. relative index § 23. Dr. Wollaston suggested a method, founded ondetermined by the? limit of total reflexion, to determine the relative in- total reflexion; , , . dices and retractive powers ot dilierent substances. Ifthe angle of incidence, <p, be measured by any device,Equation (9) will give, m = snip And thence the from which,
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