Page:Popular Astronomy - Airy - 1881.djvu/59

Rh It means no such thing, The proper word for describing that position is " vertical." Perpendicular is a relative word, and it ought not to be used without reference to something else. Vertical is an absolute word. Thus, in speaking of the word perpendicular, with reference to that trough of water or prism in the experimental case before us, I mean a line perpendicular, not to the horizon, but to the surface of the trough. I explain that particularly, because in connection with these matters, from defective education and other causes, false meanings are often given to words. The law of refraction is this: that when that beam of light represented by AB, in Figures 4 and 5, enters the side of the prism, it is bent into a position approaching more nearly to the direction of the line perpendicular to the surface. The refraction of light produced by glass or water is well understood. We know by experiments too, that air produces refraction. We apply the same laws which relate to water or glass, to the computation of refraction by atmospheric air. We find that air will alter the course of the light in such a manner, that the beams of light enter our eye more vertically than they otherwise would do; that all objects will necessarily appear to be higher than they are in reality; and applying then a proper correction by the law of refraction, based upon experiments with water or glass, we find everything properly adjusted, and that the stars revolve round one axis most accurately in the manner we have represented. These, with some additions were the main points of the subject of yesterday's lecture; I will now proceed to the lecture of to-day.

I have stated my intention of explaining how we measure the distance of the sun, and the moon, and