GEISSLER TUBES--VACUUM TUBES OF CROOKES, HITTORF AND LENARD--THE CATHODE RAY--ROENTGEN’S GREAT DISCOVERY IN 1895--X-RAY APPARATUS-- SALVIONI’S CRYPTOSCOPE--EDISON’S FLUOROSCOPE--THE FLUOROMETER--SUN BURN FROM X-RAYS--USES OF X-RAYS. The majority of people have been accustomed to regard light as something to be excluded and controlled by opaque screens just as effectively as rain is excluded by a tin roof, or cold is kept out by a brick wall. The shady retreat furnished relief from the garish day to the primitive man, and the opaque shades and Venetian blinds of modern civilization exclude the excess of light at our windows. Sunshine and shadow have, in fact, been correlated conditions to the ordinary observation of man since time began. The last few years of the Nineteenth Century, however, were to witness the discovery of a new kind of light ray which, in its behavior, subverted all previous conception of the nature and action of light. It was a species of electric light, which we are accustomed to regard as brilliant, but this light ray was invisible to the eye. It could not be refracted or bent from its course by a prism or lens, and it was so subtle, penetrating and insidious, that it could not be barred out like sunlight, but passed readily through many opaque substances, such as wood, flesh tissue, paper (even a book of 1,000 pages), as well as some of the metals. The lighter the weight of the substance, or less its density, the easier these rays passed through it, or the more transparent such bodies were to the rays. The heavier metals, like platinum, gold and lead, were practically opaque, or allowed none of the rays to pass through them, while the very light metal aluminum was about as transparent to these rays as was glass to ordinary light, and for that reason this metal could form window panes for such rays, while excluding other light. Most organic substances are transparent or semi-transparent to these rays, and hence such rays readily pass through the body of an individual, being only intercepted in part by the denser parts of the anatomy, such as the bones, so that a man in such light no longer casts a well-defined shadow of his outline, but the shadow disclosed is that of a skeleton, by virtue of the greater density of the bones. Any object of higher density, such as a ring upon the finger, clearly establishes its shadow by virtue of its greater density. Likewise, any foreign object in the body, such as a bullet from a gun-shot wound, or a foreign body accidentally swallowed, is perfectly disclosed and located by the shadow which it casts. As these light rays have been characterized as invisible, it may be difficult to understand how invisible rays can cast a visible shadow, and it should be here stated that when these unseen rays fall upon certain chemical substances the latter are made to glow with a peculiar fluorescence, and a screen made of such fluorescing materials will light up where the rays fall