LENGTHENING THE VISIBLE SPECTRUM. 89 



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 be- 

 come visible upon the white screen ; * 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 in- 

 fluence 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 ab- 

 sorbed. 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.f 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 altera- 

 tion more minutely, named it fluorescence. 



Investigations demonstrate that, besides the calcic fluoride, there 

 are an entire series of fluid substances possessing this property of con- 

 version : 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, fre- 

 quently 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 solu- 

 tions 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 fluores- 

 cing solutions, and is by them partly transmitted and partly absorbed. 



* In this experiment, the screen, impregnated with silver chloride, was replaced by a 

 white one. 



f The name of caloresccnce would be far more applicable to this peculiarity than to 

 the one mentioned further below. 



