386 Mr. E. Wilson on the Law of Dispersion. 



or, if V = unity, and /a be the refractive index, 



l=(*+*+.Q'-* w 



The wave-length of the ray, whose velocity in the refracting 

 substance is the same as that in vacuo, is determined by the 

 equation 



a + b\+ - =e^ 

 A, 



1 very nearly. 



(2) 



The ray of greatest wave-length which enters the refracting 

 substance depends upon the angle of incidence ; and, if this 

 angle be ®, is determined by the equation 



X sin © 



=\/2 very nearly 



(3) 



if the angle of incidence = 45°. 



All rays of wave-length intermediate between those deter- 

 mined by equations (2) and (3) travel faster in the refracting 

 substance than they do in vacuo. 



Professor Langley, in the following table *, gives the wave- 

 lengths, with the probable errors, of the six ultra-red lines 

 that he determines : — 



Date of observation. 



X. 



n. 



April 1, 1882 



April 9, 



l-010±0-0053 

 1-200+0-0069 

 1-658 ±0-0091 

 1-767 ±0-0094 

 2-090 ±00104 

 2-356 ±0-0110 



1-5654 



1-5625 

 1-5562 

 1-5549 

 1-5511 

 1-5478 



June 27, , 



June 13-27, „ 



July 14, „ 



June 7, , 





But repeated investigations have shown that better results 

 are obtained if three of these wave-lengths are altered within 

 the limits of probable error. 



For the Hilger prism used by Professor Langley in his 

 investigation, the constants are 



* Phil. Mag. March 1884. 



