406 Prof. S. P. Langley on Invisible Heat-spectra 



to make us inclined to proceed with caution ; but, speaking 

 with the reserves indicated by the conditions referred to, I 

 may say that we have every reason to believe that the 

 minimum wave-length assignable to the maximum ordinate 

 of the heat-curve, in the spectrum of a source whose tem- 

 perature varies from 100° to 0° Centigrade, is a little less 

 than 5/u, and a little over 6/n, and that these may be in- 

 definitely greater. This refers, it will be remarked, only to 

 the position of the maximum ordinate, while the extreme 

 portions of the curve measured on (corresponding to an index 

 of 1*45) have probably at least three times this wave-length. 

 I shall be better understood, perhaps, if I say that some of 

 the heat radiated by the soil has probably a wave-length of 

 over 150,000 of Angstrom's scale, or about twenty times the 

 wave-length of the lowest visible line in the solar spectrum, 

 as known to Fraunhofer. 



These investigations are still going forward, and I hope 

 soon to give more exact values. But I have presented the 

 present ones, though imperfect, because they give us at least 

 some knowledge of a region of which we are at present quite 

 ignorant, and because they are thus, I think, of some interest 

 both to the physicist and to the astronomer : to the physicist, 

 as showing that the wave-lengths which Newton measured to 

 the ipqq of an inch are so far from being the limits of 

 nature's scale, that the existence of measurable wave-lengths 

 of something greater than ^o °^ an mca * s rendered at 

 least highly probable ; to the astronomer, because we find 

 that the heat radiated from the soil is of an almost totally dif- 

 ferent quality from that which is received from the sun, so that 

 the important processes by which the high surface-temperature 

 of the planet are maintained can now be investigated with, 

 we may hope, fruitful results in connection with the researches 

 here described. 



I should not close this preliminary account without stating 

 that I have in these observations been throughout and at 

 every stage indebted to Messrs. F. W. Very and J. E. Keeler, 

 of this Observatory, for a collaboration without which it could 

 not have appeared in its present form. 



Addendum. 



Reduction of Observations made with the Hock-salt Prism to 

 a Standard Refracting Angle ; ivith a Table of Refractive 

 Indices of Rock-salt. 



The surfaces of the rock-salt prism and lenses undergo a 

 deterioration when exposed to the air, which is more or less 



