Because every surface has a peculiar constitution, or atomic condition, _by which the light falling upon it is influenced_. In tropical climates, where the brightness of the sun is the most intense, there the colours of natural objects are the richest; the foliage is of the darkest green; the flowers and fruits present the brightest hues; and the plumage of the birds is of the most gaudy description. In the temperate climates these features are more subdued, still bearing relation to the degree of light. And at a certain depth of the ocean, where light penetrates only in a slight degree, the objects that abound are nearly colourless. It has been held by many philosophers (and the theory is so far conclusive that it cannot be dispensed with) that there is an analogy between the vibratory causes of _sound_, and the vibratory causes of _colour_. Any one who has seen an Æolian harp, and listened to the wild notes of its music, will be aware that the wires of the harp are swept by accidental currents of air; that when those currents have been strong, the notes of the harp have been raised to the highest pitch, and as the intensity of the currents has fallen, the musical sounds have deepened and softened, until, with melodious sighing, they have died away. No finger has touched the strings; no musical genius has presided at the harp to wake its inspiring sounds; but the vibration imparted to the air, as it swept the wires, has alone produced the chromatic sounds that have charmed the listener. If, then, the varied vibrations of the _air_ are capable of imparting dissimilar sensations of _sounds_ to the _ear_, is it not only possible, but probable, that the different vibrations of _light_ may impart the various sensations of _colours_ to the _eye_? CHAPTER XXIII. 472. _What is the refraction of light?_ When rays of light fall _obliquely_ upon the surface of any _transparent medium_, they are slightly diverted from their course. This alteration of the course of the rays is called _refraction_, and the degree of refraction is influenced by the difference between the _densities_ of the mediums _through which light is transmitted_. [Verse: "Let your light so shine before men, that they may see your good works, and glorify your father which is in heaven."--MATT. V.] 473. _If a ray of light falls in a straight line upon a transparent surface, is it then refracted?_ In that case the ray pursues its course--_there is no refraction_. 474. _Is the direction in which the rays are bent, or refracted, influenced by the relative densities of the media?_ A ray of light falling slantingly upon a _window_, in passing through it is slightly brought to the _perpendicular_; and if it then falls upon the surface of water, it is still further brought to the perpendicular in _passing through the water_.