. Éléments d'astronomie théorique et descriptive, pour l'utilisation des collèges et académies. henomena sont remarqué,et les mêmes lois tenir bon, si les médias sont séparés des surfaces. bycurved 50. La Réfraction astronomique,--il byexperimentthat est déterminée la densité de l'air diminue graduellement à mesure que nous ascendabove la surface de la terre, et on estime qu'à adistance de cinquante milles au-dessus de la surface de la limite supérieure de lescolliers ; ou, en tout état de cause, que la densité de l'air est sosmall à cette distance qu'elle exerce une refractingpower non appréciable. Nous pouvons donc envisager de t

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. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. henomena are noticed, and the same laws hold good, if the media are separated bycurved surfaces. 50. Astronomical Refraction, —-It is determined byexperimentthat the density of the air gradually diminishes as we ascendabove the surface of the earth, and it is estimated that at adistance of fifty miles above the surface the upper limit of theair is reached; or, at all events, that the density of the air is sosmall at that distance that it exerts no appreciable refractingpower. We may, therefore, consider the air to be made up ofi series of strata concentric with the earths surface, the ^thick-ness of each stratum being in-definitely small, and the den-sity of each stratum beinggreater than that of the stra-tum next above it. Now, inFig. 20, let the arc BD repre-sent a portion of the earthssurface, and the arc MN a por-tion of the upper limit of theatmosphere. Let 8 be a ce-lestial body, and SA a ray oflight from it, which enters theatmosphere at A. Let themg2Si normal (or radius) AC be. REFRACTION 53 drawn. As the ray of light passes down through the atmos-phere, it is continually passing from a rarer to a densermedium, so that its path is continually changed, and becomesa curve AL, concave towards the earth, and reaching the earthat some point L. Since the direction of a curve al any pointis the direction of the tangent to the curve at that point, theapparent direction of the ray of light at L will be repre-sented by the tangent LS, and in that direction will the body5 appear to lie, to an observer at L. If the radius CL beindefinitely prolonged, the point Z, where it reaches the celestialsphere, will be the zenith of the observer at L, the angle ZLSwill be the apparent zenith distance of the body S, and theangle which the line drawn from the body to the point Lmakes with the line LZ will be the true zenith distance of SThe effect, then, of refraction is to decrease the appar

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