180 THE ROYAL SOCIETY OF CANADA 



region, that considerable divergence exists in their results. The first 

 recorded investigations were somewhat cursory attempts by Snow 

 and by Drew and it was not until 1903 that we have any results in 

 which confidence can be placed. 



These are due to Coblentz and Geer^ who worked with a rock 

 salt prism spectrometer and a radiometer and found three definite 

 lines between -97 fj, and 1 • 285^. In addition to these they were able 

 to identify six lines in the neighbourhood of 5 • O/x and possibly one other 

 near 3 • 0/x. Coblentz^ repeated this work a couple of years later and 

 announced that there are no important lines beyond 1 • 3fx except 

 those near 5-0)U. W. J. H. MolF somewhat later using a rock salt 

 spectrometer and thermopile in connection with an automatic record- 

 ing device identified five lines between 1 • O/j. and 1 • T/x. In direct 

 opposition to the results of Coblentz and Geer, Moll states that there 

 is no measurable emission above 1 • 7/x. Probably the most accurate 

 measurements on the infra-red spectrum of the mercury arc are those 

 made by Paschen* with a concave grating and a Rubens thermopile. 

 By means of the better definition and the higher dispersion afforded 

 by the grating, Paschen was able to separate maxima which had 

 previously been recorded as single lines. In all he identified fourteen 

 lines between 1 • 0/x and 1 • 7/i and he confirmed the statement by Moll 

 that there are no lines beyond 1 • 7/x. He^ subsequently repeated his 

 measurements and found a maximum at 4- 0/x; but inasmuch as this 

 maximum came out in the arc spectrum of a number of the elements 

 he concluded that it was due to the presence of hydrogen. In these 

 later measurements a bolometer was used in combination with a 

 grating. More recently still H. Rubens and O. von Baeyer^ have 

 succeeded in showing that the mercury arc emits a radiation of wave- 

 length about 313/x. They succeeded in isolating this radiation by the 

 method of focal separation previously used by Rubens and Wood^ 

 and in measuring its wave-length by means of a Fabry and Perot 

 interferometer of a special type in combination with a Rubens micro- 

 radiometer. Subsequent measurements^ by them on this radiation 



iW. Coblentz and W. C. Geer, Phys. Rev., 16 pp. 279-286, 1903. 



^Coblentz, Phys. Rev., 20 pp. 122-124, 1905. 



3Moll. Kon. Akad. Wet. Amsterdam. Proc. 9 pp. 544-548, 1907. 



^Paschen, Ann. der Phys. 27, 3, pp. 537-570, 1907. 



sPaschen, Ann. der. Phys. 33, 4, pp 717-738, 1910. 



6H. Rubens and O. von Baeyer, Phil. Mag., 21 pp. 689-695, 1911. 



^H. Rubens and R. W. Wood, Preuss. Akad. Wiss. Berlin, Sitz. Ber. 52. pp. 

 1122-1137, 1910. 



«H. Rubens and O. von Baeyer, Preuss. Akad. Wiss. Berlin, Sitz. Ber. 30 pp. 

 666-667, 1911. 



