42 H. Rubens and B. W. Snow on the Refraction of 



mentioned one in particular, that, from a definite wave- 

 length on, the indices of refraction would assume negative 

 values, which points to an utter impossibility. There re- 

 mained, however, the possibility that within the limits of 

 Professor Langley' s measurements of energy, the straight hue 

 exterpolation gave results which were at least a first approxi- 

 mation to the true value. A glance at the curve (fig. 4, a) 

 shows, on the other hand, that in reality this is not the case. 

 Indeed it is true that our own curve of dispersion tends 

 toward a straight line until a point is reached almost as distant 

 as X = 5/ul ; but at \ — 5fi the curve begins gradually to lessen 

 its inclination to the horizontal axis of wave-lengths ; and at 

 X = S/jl tliQ effect of this curvature is so considerable that a 

 straight line exterpolation carried from X = ba on to this 

 point would introduce an error in the determination of wave- 

 length, not less than 1/jl. 



In the following Table a comparison is made between our 

 results and those of Professor Langley. It is to be noticed 

 that his curve, as far as this is plotted from his observations, 

 agrees fairly "well with our own, but that his values given by 

 exterpolation differ widely from those observed by us, and 

 that their difference increases as the wave-lengths become 

 longer. There are also added here for completeness the data 

 obtained from the previous paper. That an easier com- 

 parison may be made with Langley's figures, the wave-lengths 

 selected increase by multiples of X. D = 0'589/a. 







Table II. 









Wave-length. 



n 

 (Langley). 



n 



(Rubens). 



n (Rubens & 

 Snow). 



n (Langley) - 

 w(Rub.&Sn.). 



1. X D =0-589 



1-5442 > 





1-5441 



1-5441 



o-oooi 





2. X D = 1-178 



1-5301 





1-5300 



1-5301 









3. X d = 1-767 



T5272 





1-5269 



1-5272 









4. X d =2-356 



1-5254 



o 



1-5253 



1-5256 



-00002 





5. X^= 2-945 



15243 



-5 



1-5241 



1-5240 



+0-0003 





6. X D =3-534 



1-5227 





1-5227 



1-5226 



1 





7. X D =4-123 



1-5215 





1-5214 



1-5212 



3 





8. X D =4-712 



1-5201 





1-5202 



1-5200 



1 



9. X I) =5-301 



1-5186 





1-5189 



1-5188 



-0-0002 





10. X D =5-890 



1-5172-j y 





L5177 



5 





11. X D =6-480 



1-5158 | | 





1-5166 



8 





12. X D =7 070 



1-5144 v.! 

 1-5129 1 





1-5157 



13 





13. X D = 7-66 





1-5148 



19 



14. X D =8-25 



1-51 15 J P* 





1-5138 



23 







