154 ASTRONOMICAL REFRACTION. SECT. XVIII. 



manner as if the atmosphere consisted of an infinite number of 

 strata of different densities. The object is seen in the direction 

 of a tangent to that part of the curve which meets the eye ; con- 

 sequently the apparent altitude (N. 190) of the heavenly bodies 

 is always greater than their true altitude. Owing to this cir- 

 cumstance, the stars are seen above the horizon after they are 

 set, and the day is lengthened from a part of the sun being 

 visible, though he really is behind the rotundity of the earth. 

 It would be easy to determine the direction of a ray of light 

 through the atmosphere if the law of the density were known ; 

 but, as this law is perpetually varying with the temperature, 

 the case is very complicated. When rays pass perpendicularly 

 from one medium into another, they are not bent; and ex- 

 perience shows, that in the same surface, though the sines of 

 the angles of incidence and refraction retain the same ratio, 

 the refraction increases with the obliquity of incidence (N. 

 189). Hence it appears that the refraction is greatest at the 

 horizon, and at the zenith there is none. But it is proved 

 that, at all heights above ten degrees, refraction varies nearly 

 as the tangent of the angular distance of the object from the 

 zenith, and wholly depends upon the heights of the barometer 

 and thermometer. For the quantity of refraction at the same 

 distance from the zenith varies nearly as the height of the baro- 

 meter, the temperature being constant ; and the effect of the 

 variation of temperature is to diminish the quantity of refraction 

 by about its 480th part for every degree in the rise of Fahren- 

 heit's thermometer. Not much reliance can be placed on celestial 

 observations, within less than ten or twelve degrees of the horizon, 

 on account of irregular variations in the density of the air near 

 the surface of the earth, which are sometimes the cause of very 

 singular phenomena. The humidity of the air produces no sen- 

 sible effect on its refractive power ; and it has been proved that 

 the amount of refraction is the same whatever be the velocity of 

 the incident light, that is whether the light comes from a star in 

 that part of the heavens towards which the earth is going, or 

 from one in that part of the sky whence it is receding. 



Bodies, whether luminous or not, are only visible by the rays 

 which proceed from them. As the rays must pass through strata 

 of different densities in coming to us, it follows that, with the 

 exception of stars in the zenith, no object either in or beyond our 



