Page:Popular Science Monthly Volume 23.djvu/102

92 parallel through a condensing lens, passes through a square glass vessel containing clear water. We introduce a pane of violet glass into its path. The illuminating radiation is cut off by it, and only a few violet rays are transmitted. The ultra-violet ones, however, are represented much more abundantly in the now invisible pencil. Their existence is revealed at once when we add a few drops of the quinine solution to the water. Bright blue-colored clouds now move within the vessel, generated by the quinine absorbing the ultra-violet rays and changing them into blue light. The appearance becomes still brighter by substituting the more energetic fluorescent esculine in place of the quinine.

If we draw a sketch upon yellow paper with an esculine solution, it is invisible in daylight as well as by electric light. But if we insert the violet glass into the pencil-cone, the single parts of the picture begin to shine vividly with a blue light. The sketch flames up at once in the obscurity before our eye, and we might imagine that we have been transported into Fairy-land.

We have until now had our attention engaged with the ultra-violet rays; it remains to speak of their practical adaptation. On account of their chemical effect upon the salts of silver, they constitute the basis of an important branch of industry—photography. As we have seen above, the red rays have almost no influence upon such preparations, while the effect of the yellow and green ones, when compared to that of the blue, violet, and ultra-violet, is not very great. Many mysteries of photography, incomprehensible to the layman, are explained hereby. A red and a black dress, for instance, are exactly alike upon a photograph, while blue and white, in their effect, approach nearer to white.

We now turn to the opposite end of the spectrum—the ultra-red rays. Our proposition is, "Are we able to render perceptible to the eye, the organ of sight, those rays that operate upon our sense of feeling simply as conveyers of heat?" We can attain our purpose only by augmenting the number of vibrations the thermal rays, by their influence upon suitable bodies, in such a manner that they are rendered perceptible to the visual organ. We provide the electric lamp with a parabolic reflector, A B, silvered and polished within, with incandescent carbon-points in its focus. The intense rays of the lamp are made parallel by the reflector, and pass through the room as a bright horizontal column. We recognize their course by the illuminated dust-particles of the air. We interpose another spherical reflector, C D, also silvered and polished, in the course of the rays. According to the law of reflection, all the rays falling upon the latter unite into one point, the focus (Fig. 3). It is easily recognized, since it brightly illuminates the dust-particles of the air. But not