Page:The fairy tales of science.djvu/122

94 upon the screen above the maximum point of the heat rays. Lastly, we find that actinism is more refrangible than either heat or light, as we know that the maximum of this power is found in the upper part of the spectrum at a point where light rapidly diminishes, and where scarcely any heat can be detected. The analysis of the sunbeam by means of the prism must excite our wonder. Who could imagine that a simple wedge-shaped piece of glass would be able to separate those imponderable agents which reach us after having travelled ninety-five millions of miles together? We can isolate either of these solar principles without the aid of a prism. The crystal called black mica does not admit light to pass through it, but it is freely penetrated by heat; and on the other hand, glass stained green by oxide of copper, offers scarcely any impediment to the passage of light, though it effectually stops the rays of heat. Again, a yellow transparent glass obstructs the chemical radiations, while a dark blue medium, which arrests nearly all the light, allows them to pass. Thus we see that the physical conditions of the three solar principles are essentially different. Let us now consider the magic influences of this sunbeam over the animal and vegetable kingdoms. The luminous principle first demands our attention; for although we are told that light is less abundant than either heat or actinism, we cannot help