92 THE POPULAR SCIENCE MONTHLY. 



parallel through a condensing lens, passes through a square glass ves- 

 sel 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 repre- 

 sented much more abundantly in the now invisible pencil. Their ex- 

 istence 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 * uj^on yellow paper with an esculine solution, 

 it is invisible in daylight as well as by electric light. But if we in- 

 sert the violet glass into the pencil-cone, the single parts of the pic- 

 ture 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 com- 

 pared 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, ap- 

 proach nearer to white. 



We now turn to the opposite end of the spectrum the idtra-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 elec- 

 tric 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 



* The one employed by Mr. W. Fried, of Augsburg, represents a Renaissance ornamen- 

 tation, of about sixty centimetres in diameter. 



