The infra-red world is as strange as the ultra-violet. The sky appears black, foliage a beautiful rich red, and there are long, heavy shadows
With light it is the same. There are octaves of light which our eyes can never hope to see. Perhaps the best known of invisible rays are those used in wireless telegraphy; they arc produced by vibrations of far lower frequency than those which we see as sunlight.
When you strike the middle "C" on a piano you hear a single musical note. And so, when you look at the world about you through a pane of red glass, you see things in a single light-note, as it were. Change the color of the glass and the world appears different. The same trees, the same flowers, the same houses are there, but with one color details are obscured and with another intensified.
It is perfectly possible to view the world with invisible rays and to learn things about which we never dreamed of in our philosophy—only we must use an eye, which, unlike our own eyes, will see the unknown world for us and make a picture of it which we can perceive. The ordinary photographic camera is such an eye. The sensitized plate is extraordinarily responsive to those very high-pitched vibrations that do not affect the eye. All that remains is to strike the single note in a given octave of light, with which the world is to be viewed in order to see things as they are but as we never see them.
In order to see the world with invisible ultra-violet rays something better than glass must be employed; for glass is almost as opaque to them as a plate of sheet-iron. Quartz must be used, since quartz is transparent to them. Hence a quartz lens must be fashioned for the camera. To exclude all but violet rays from the lens a filter must be employed—a kind of sieve through which only the ultra-violet rays will pass, just as only red rays will pass through red glass. Some fifteen years ago I discovered that an aniline dye, called nitroso-dimethyl-aniline, would exclude all but the ultra-violet rays, the effect of which I wished to study. Thin films of silver are also serviceable, as well as the vapor of