Abstract
In the second part of his prize-winning essay of 1729, Bouguer (1698–1758) considers a light ray in a spherically symmetric medium and introduces the invariant named after him. He deduces an expression of the refraction integral, leading to a table which is computed for a particular atmospheric model. He then studies the influence of the ray’s curvature on the horizon dip and predicts the hillingar effect. As a complement, we recall that the Bradley formula can be derived from the Bouguer model. This helps to understand the strong convexity of horizontal refraction versus refractivity.
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