88 THE POPULAR SCIENCE MONTHLY. 



LENGTHENING THE VISIBLE SPECTKUM. 



By JOHANNES GOTZ. 



THE phenomena of refraction and dispersion teach us that a body 

 in a state of intensest heat emits not alone powerful thermal 

 rays, but also all possible sorts of light (luminous colors). Diffraction 

 convinces us that radiation is a wave-motion of an extremely fine, 

 elastic, fluid medium, ether, and at the same time it enables us to 

 compute the wave-length of the single rays. As is known, our ap- 

 paratus suffices for taking cognizance of from one hundred and sixty 

 to seven hundred and ninety billion undulations of heat or light per 

 second, while all the vibrations of the ether lying either below or 

 above are withdrawn from our direct observation. It is the purpose 

 of this article to show what ways and means have been found for 

 rendering the latter rays at least partly visible to our eye. 



We throw a spectrum upon a white screen by means of a prism. 

 The rays of the inferior number of undulations (ultra-red) will lie be- 

 yond the red, the superior (ultra-violet) beyond the violet end of the 

 spectrum. We will begin with the latter. 



We replace the screen, generally covered with ordinary white paper, 

 by another one, the covering of which is impregnated with silver chlo- 

 ride, a combination of the two elements, chlorine and silver.* When the 

 light has for some time operated upon this preparation, we interrupt 

 it, and examine the screen by the light of a candle. We find that the 

 coating has become blackened ; that the blackening is insignificant at 

 the place where we formerly had red light, but that it increases the 

 nearer we approach toward the violet end ; that it finally attains its 

 maximum beyond this place, and gradually grows weaker until, at a 

 certain distance, it disappears from the violet end. Whence this black- 

 ening ? By the operation of the ether-waves, the combination of 

 chlorine and silver was dissolved, the chlorine passed into the air in 

 the form of vapor, while the silver was precipitated in microscopically 

 fine pearls upon the paper. The coating became black, because silver 

 is not metallically lustrous in such minute division, but simply consti- 

 tutes a black powder. 



This experiment convinces us that rays will still be found beyond 

 the violet end of the spectrum, which, on account of their high number 

 of undulations, are shrouded from our sight, and yet betray their pres- 

 ence by the decomposition of silver preparations. These rays have 

 been called actinic or chemical rays, and their spectrum the chemical 

 spectrum. 



* This and most of the following experiments succeed well only with a very great 

 power of light. It is necessary, therefore, to sustain the lamp by a battery of from sixty to 

 eighty elements. In the present case, sixty-four large Bunsen elements were employed. 



