Rays of Great Wave-length in Rock-salt, Sylvite, Sfc. 43 



The values, therefore, attributed by Professor Langley* to 

 the wave-lengths in that region of the spectrum lying be- 

 tween \ = and \ = b/jb are undoubtedly correct. Beyond 

 this limit, however, at least as far as \ = 8'3/j,, the values 

 assumed are too small, but it is not impossible that when still 

 greater wave-lengths are reached the sign of the error may 

 change. The results, nevertheless, of his observations remain 

 of the greatest interest, since it will be easily possible to apply 

 a correction to the wave-lengths as soon as the dispersion 

 in rock-salt can be followed to sufficiently small indices of 



Sylvite. 



The behaviour of rock-salt is in every respect similar to 

 that of the mineral sylvite, to which it stands in close chemical 

 relation. There was placed at our disposal a prism of this 

 material, 14 millim. at the base and 20 millim. in height. 

 The surfaces of this prism were so well polished that the 

 refracting angle could be determined to within 0*5 minute. 



In fig. 2 the observed galvanometer-deflexions are plotted 

 as functions of the angular deviation of the arm of the bolo- 

 meter. From this curve is computed the table of dispersion 

 in the manner described above. Corresponding to this is 

 plotted the curve of dispersion (fig. 4, b) . 



Table III. 



Refracting Angle of the Prism of Sylvite, = 59° 54'. 

 K = 8-022 p ; a x is 10th Order. 



Name. \ a. 



I 



5* 



F . 

 D. 

 C . 



a x . 



\- 

 a 



a 3 . 



a,. 



K- 



37 30 



1-5048 



36 55 



1-4981 



13J 



1-4900 



35 57 



1-4868 



37 



1-4829 



32 



1-4819 



27 



1-4809 



23^ 



1-4802 



20 



1-4795 



16* 



1-4789 



13 



1-4782 



10 



1-4776 



7* 



1-4771 



434 



486 

 589 

 656 

 802 

 845 

 893 

 944 

 003 

 070 

 145 

 '234 

 •337 



Name. 



a 



t 



a 

 b 8 . 



c \ • 

 Ob. 



C 2 ■ 

 V 



a 10 



60 D 



34 59j 

 56£ 



53 



48 



43 



40* 



38~ 



35^ 



32 



28 



0,9 



1-4766 

 1-4761 

 1-4755 

 1-4749 

 1-4742 

 1-4732 

 1-4722 

 1-4717 

 1-4712 

 1-4708 

 1-4701 

 1-4693 

 1-4681 



\. 



458 

 603 

 781 

 005 

 291 

 673 

 ■209 

 ■561 

 Oil 

 577 

 345 

 412 

 022 



* Langley, Sill. Joum. [3] xxxi. pp. 1-22, 1886 ; further [3] xxxii. 

 pp. 83-106, 1886, and [3] xxxviii. pp. 421-440 ; Phil. Mag-, xxvi. p. 505, 

 1888. The same is true of t he paper of Angstrom, Ofversigt af Konql. 

 Vet.-Akad. Forhmdl. ix. p. 549, 1889, and vii. p. 331, 1890, and \V. 

 II. Julius, Arch, Neerl. i. pp. 310-384, 1888. 



