Page:The fairy tales of science.djvu/119

 through a bent tube, nor turn a corner. Bodies through which light passes freely are called transparent, and those which do not admit it to pass, opaque. When light falls upon an opaque surface a portion is absorbed and another portion reflected; when the reflected portion is considerable the surface appears white, and black when the portion is inappreciable.

We have said that a sunbeam consists of three great principles, namely, light, heat, and actinism. These may be but modifications of one force, but in the present state of our knowledge it is better to regard them as distinct agents.

Light acts upon the organs of vision, and enables us to distinguish external objects. Heat regulates the solid, liquid, and aëriform states of matter, and maintains this planet in the condition which is essential to the well-being of its inhabitants. Actinism brings about those wonderful chemical changes which are constantly occurring in nature. These three principles unite to form our magic sunbeam, just as three chemical elements unite to form a compound.

How can we decompose a sunbeam? How can we separate those principles which are linked together in such a mysterious manner? Easily enough, for by the instrumentality of a triangular bar of transparent glass, called a prism, the beam can be instantly resolved into its components.

If a sunbeam, admitted into a dark chamber by