Our proposition is, "May not these rays, even if only partially, be rendered visible to the eye?" Let us, for this purpose, shut out all the other vividly luminous colors of the spectrum, so that they shall not interfere, by their excess of illumination, with the feeble effect of the ultra-violet rays. We interpose a black screen in their path, and cut off all except the extreme violet ones. The ultra-violet rays now become visible upon the white screen;[1] we see them of a feebly lustrous lavender-gray.
This method of rendering the ultra-violet rays visible is extremely primitive. Stokes, the successor of the great Newton in the professor's chair of the University of Cambridge, indicated a means by which our object is attained much more effectively. He introduced a piece of calcic fluoride into the ultra-violet part of the spectrum, and found that this crystal began to shine brightly with a blue light. Before we attempt to elucidate this peculiarity, however, let us consider the influence of the motion of ether upon a body.
When a ray arrives upon the surface of a body, three things may be imagined: The ray is either reflected, or it is transmitted, or absorbed. We describe the first two cases as reflection and refraction. In the third case, the ray is absorbed, and serves for heating the body, which itself emits again the arriving motion of ether in the form of calorific rays.[2] Besides these three cases, another, a fourth one, is possible, to wit, that, although the arriving rays are absorbed, they are not wholly employed in the heating of the body, but are partly altered into rays of another number of waves, and are emitted again under their changed form. Stokes, who first investigated this alteration more minutely, named it fluorescence.
Investigations demonstrate that, besides the calcic fluoride, there are an entire series of fluid substances possessing this property of conversion: for instance, petroleum; again, the solution of the highly esteemed febrifuge—quinine sulphate; esculine, an extract of the bark of the common horse-chestnut; leaf-green, or chlorophyl; eosine, frequently used in the manufacture of red ink; and, finally, in a high degree, the solution of a substance discovered by Bayer, of Munich, fluoresceine (resorcinphtaline).
Let us get better acquainted with these fluorescing substances. Best for this purpose is a narrow glass tube, filled with rarefied air—a so-called Geissler's tube, surrounded by an envelope, containing solutions of such substances in four divisions (Fig. 1).
By means of the electric current we bring the inclosed air to a red heat. It emits whitish-violet light, which penetrates into the fluorescing solutions, and is by them partly transmitted and partly absorbed.
