28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



" We commenced by using an electric arc with carbons 12 mm in 

 diameter in the position indicated. These were supplied by an engine 

 of three horse-power ; but even in this case the pit of the crater did 

 not nearly cover the very short slit (its length is 8 mm). For these 

 last and most difificult measurements, we have been obliged to pro- 

 cure the use of an engine of twelve horse-power and carbons 25 mm 

 (one inch) in diameter. With this enormous current the hottest part 

 is not easily maintained in place. To keep it directly in front of the slit 

 we have tried various plans, such as boring out the carbons length- 

 wise, so as to form hollow cylinders of them, and filling the core with 

 a very pure carbon tempered to the requisite solidity. Ordinarily it 

 will be sufficient however to first form the central crater by a drill. 

 This gives us a persistent crater, whose light, in the position shown 

 in the engraving, filled a slit whose vertical height is 8 mm. It is prob- 

 ably the intensest artificial heat ever subjected to analysis. 



" In the following brief table we have summarized the results of 

 all this labor. Our working method gave the index in terms of the 

 wave-length, but since ordinarily the former is the known, and the 

 latter the unknown quantity, we here give the mean probable error as 

 finally corrected as a function of the latter. 



Given indices of „, , , r ,• , • 



refraction in Wave-lengths from direct observation 

 rock-salt (a) by the eye (fc) by the 



prism bolometer 



1.5442 ^Ds = o/'.sSgo ± 0.000 (a) 



1.5301 2 X ^Da = I -1780 ± 0.002 (b) 



1.5272 3 X ^Da = I .7670 ± 0.005 (b) 



1.5254 4 X ?^D2 = 2 .3560 ± 0.009 (b) 



1.5243 5 X XD^ = 2 .9451 ± 0.013 (b) 



1.5227 6 X XD2 = 3 .5341 ± 0.019 (b) 



1.5215 7X ^0-2=^ 4 .1231 ±0.029 (b) 



1.5201 8 X ^■Da = 4 7121 ± 0.043 (b) 



1.5186 9X^02^=5 .3011 ±0.065 (b) " 



Compared to our later determinations and those of Paschen, these 

 observed indices of refraction of rock salt are found to differ but one 

 or two units in the fourth place of decimals from the true values. 

 To estimate the wave lengths of his lunar spectrum, Langley extra- 

 polated, using the best formula then available. As this formula was 

 erroneous for these great wave lengths, its results gave him exag- 

 gerated impressions of the greatness of the wave lengths he actually 

 observed. For instance, in Appendix No. i of his paper " The Solar 

 and Lunar Spectrum," he gives a wave length as 21.5 microns which, 

 corrected by modern data, should read 10.7 microns. Similarly the 



