THE EXTREME INFRA-RED RADIATIONS. 1 



By 0. E. Guillaume. 



If we compare our present knowledge of the spectrum with what was 

 known even as recently as ten years ago, we can but marvel at the 

 progress that has been made in that brief period. In 1888 the ultra- 

 violet spectrum had been investigated by M. Cornu, but only as far as 

 the point at which the air totally absorbs the rays. In the infra-red, 

 the region actually measured did not extend below 2 or 3 microns, 

 although Langley had explored for some distance beyond. 2 Electrical 

 waves were then making their first appearance and scientists were far 

 from being in accord as to their true nature, few then admitting them 

 to be identical in fundamental character with light. 



To-day the spectrum has been continued a whole octave toward the 

 shorter wave lengths by means of experiments conducted in vacuo. 

 The X rays and the uranium rays, or secondary rays derived from X 

 rays, have still further extended the ultraviolet spectrum through 

 regions not yet measured. Electrical oscillations, which from their 

 various properties have been shown to belong in the octaves below 

 those of light, have now been produced with wave lengths of about 3 

 millimeters; while the infra-red proper — that is, the direct continuation 

 of the spectrum beyond the visible ending in the red — has extended 

 itself more and more, bridging over the unknown region toward the 

 electrical waves. 



Progress in this infra-red region, after having been singularly slow 

 for several years, has recently become very rapid. Extremely merito- 

 rious researches have added largely to the older work, and have led to 

 results of the highest interest in the knowledge of the vibratory 

 motions of the ether, which we still continue to call light, although, tor 

 the physicist, those radiations which are visible form but a minute por- 

 tion of the whole. 



Our readers will recall that the infra-red radiations are revealed to 

 us only through their transformation into heat at the surface of some 

 suitable receiver. This is often a thin strip of platinum or iron or 



1 Translated from La Nature, No. 1325, pp. 332-334, October, 1898. 

 2 Translator's note: This statement is slightly in error. The wave-length measure- 

 ments of Langley in 1886 extended as far as 5.3 microns. 



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