Mr. E. Wilson on the Laiv of Dispersion. 387 



a = -6299284, 

 b = -0058580, 

 c = -0075571, 

 h = -0071680. 

 With these constants the correspondence between observa- 

 tion and calculation is given in the following table: — 



1 



Ray. 



Observed. 



ft. 



Calculated. 



Difference. 



H, 



1-607000 1-606923 

 1-589900 1-589955 

 1-556200 1-586233 

 1-579800 1-579800 

 1-575700 1-575708 





000077 

 000055 

 000033 

 000000 

 000008 

 000008 

 000021 

 000050 

 000056 

 000005 

 000060 

 000058 



F l 



b t 



D, 



c 1 :. 



A 



M-010 



£1-1931 ... 

 HL-6671 ... 

 nl-7764 ... 



^2-090 



"2356 



1-571400 

 1-565400 

 1-562500 

 1-556200 

 1 -554900 

 1-551100 

 1-547800 



1-571392 

 1-565379 

 1-562450 

 1-556256 

 1 -554905 

 1-551040 

 1-547742 



If X M =i be the wave-length of the ray which travels as fast in 

 the Hilger prism as in vacuo, and \ G be the wave-length of 

 the greatest ray which would enter the prism at an angle of 

 incidence of 45°, then 



Ht=D= 63-5 



X G =133-8 

 The only indices of refraction which have come to hand of 

 sufficient accuracy to test a formula, are those of seven prisms 

 determined by Dr. Hopkinson (Proc. R. S. June 14, 1877), 

 and given by Professor Everett in ' Units and Physical 

 Constants/ pp. 70 and 71, 1st edition ; the results are as 

 follows : — 



Hard Crown Glass. Density 2'48575. 



Ray. 



ft. 

 Observed. 



fi. 



Calculated. 



Difference. 



H, 



G 



1-532789 

 1-528348 

 1-523145 

 1-520324 

 1-517116 

 1-514571 

 1-513624 

 1-511755 



1-532789 

 1-528375 

 1 -523147 

 1-520329 

 1-517119 

 1-514576 

 L -513642 

 1-51176] 



•oooooo 



•000027 

 •000002 



•000005 

 •000003 

 000005 

 •000018 

 000006 



F 



E 



? 



B 



A 





