Page:Herschel - A Preliminary Discourse on the Study of Natural Philosophy (1831).djvu/44

 give an example:—every body knows that objects viewed through a transparent medium, such as water or glass, appear distorted or displaced. Thus, a stick in water appears bent, and an object seen through a prism or wedge of glass seems to be thrown aside from its true place. This effect is owing to what is called the refraction of light; and a simple rule discovered by Willebrod Snell enables any one to say exactly how much the stick will be bent, and how far, and in what direction, the apparent situation of an object seen through the glass will deviate from the real one. If a shilling be laid at the bottom of a basin of water and viewed obliquely, it will appear to be raised by the water; if instead of water spirits of wine be used it will appear more raised; if oil, still more:—but in none of these cases will it appear to be thrown aside to the right or left of its true place, however the eye be situated. The plane, in which are contained the eye, the object, and the point in the surface of the liquid at which the object is seen, is an upright or vertical plane; and this is one of the principal characters in the ordinary refraction of light, viz. that the ray by which we see an object through a refracting surface, although it undergoes a bending, and is, as it were, broken at the surface, yet, in pursuing its course to the eye, does not quit a plane perpendicular to the refracting surface. But there are again other substances, such as rock-crystal, and especially Iceland spar, which possess the singular property of doubling the image or appearance of an object seen through them in certain directions; so that instead of seeing one object we see two, side by side, when such a crystal or spar is interposed be-