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 metallic mirrors so as to combine reflexion and refraction. We understand he has been able to give great perfection to his instrument by calculating the forms of the lenses so as to correct all the aberrations.

177. simplest of these appearances are caused by the refraction which the solar light undergoes in passing through the atmosphere.

When a ray of light enters successively several media of different densities it suffers a refraction greater or less at each surface, as represented in Figs. 195 and 196.

It is evident that the greater the number of media and the less their individual thickness, the more continuous is the course of the light, and that if the number be infinite and thickness of each evanescent, this course must be a curve.

Now this is just the case with the atmosphere; for the air being an elastic fluid, is compressed and condensed by its own weight, so that its density increases continually from its extreme bounds to the surface of the earth; and, in consequence of this, all distant objects, whether celestial or terrestrial, except those immediately over-head, are seen by curved rays, and consequently referred by the spectator to places which they do not really occupy.

This is represented on an exaggerated scale in Fig. 197: the actual error or refraction in the apparent height of an object on the Earth varies according to General Roy, from $1⁄3$ to $1⁄24$ of the altitude; Bouguer makes it $1⁄9$, Maskelyne $1⁄10$, Lambert $1⁄14$.

The refractive power of the air varies of course with its density, and that with its temperature, for heat necessarily rarifies it. It is also subject to great irregularities from the aqueous vapours suspended in it.