2 S. P. Langley — Observations on Invisible Heat-Spectra. 



ignorance of certain physical data which have never yet been 

 obtained. 



While the general question for the physical astronomer then, 

 is " What kind of transformation does the solar energy suffer 

 at the surface of any planet?" we here seek a reply to the 

 simpler preliminary one, "What are the wave-lengths of heat 

 from non-luminous sources, such as the soil of this planet?" a 

 question which has never been answered, because there have 

 been no means of recognizing this heat when drawn out into a 

 spectrum ; indeed we so habitually associate the idea of a spec- 

 trum with that of light, that there is a certain strangeness, at 

 first in the idea even, of a " spectrum " formed by a cold body 

 like, for instance, ice. Yet the ice surface must not only be 

 capable of radiating heat to a still colder body, but according 

 to our present conceptions of radiant energy, be capable of 

 giving a spectrum, whether we can recognize it or not. It is 

 the object of the present paper to describe the actual formation 

 of such spectra, and the recognition of their heat in approximate 

 terms of wave-lengths. 



To distinguish betweeu these new regions of research and 

 the older ones, let us briefly summarize our actual information 

 about wave-lengths, since on the latter the whole question 

 largely turns, and each extension of it, we may agree, is a step 

 toward an interpretation of everything about the constitution 

 of the universe which radiant energy may have to tell us. 

 Thus there is no exact relation known between the periods of 

 vibration of certain molecules in the sun and the angles through 

 which the rays announcing them, are refracted by a prism, 

 while the wave-lengths of these rays, if known, are capable of 

 giving us quite other intelligence. 



Yet our knowledge even of the wave-lengths of light is com- 

 paratively recent, since it was only at the beginning of this 

 century that the labors of Thomas Young brought the undu- 

 latory theory itself from the disfavor in which it had lain, and 

 the memoirs in which Fraunhofer gave the first relatively full 

 and accurate measures of the wave-lengths of light date no 

 farther back than 1814. 



The measures of Newton, interpreted in terms of the present 

 theory, gave the length of the extreme violet waves at 

 tmVV.ooo oi an inch, and of the extreme red at T o,o 2 o 6 o 6 o o o> or 

 in' millimeters 0'00042 mm and 0'00067 mm respectively, numbers 

 nearly corresponding with the lines H and B, while Fraunhofer's 

 own values are comprised between 0-00036 mm and 0'00075 mm . 

 More recently the range of vision has been still more extended, 

 by the use of the fluorescent eyepiece of Soret, while by the 

 aid of photography and the employment of quartz trains, solar 

 radiations of a wave-length of about - 29// have been observed,* 



* (l-0u = 0-001 mm ).] 



