A THEORY OF THE DISPERSION OF LIGHT. 



71 



Observations on the above Results, 



In the case of oil of cassia the accordance in the values of p appears sufficiently 

 close ; especially considering that the experimental data can only be regarded as ap- 

 proximations, as fully appears from my paper on the determination of the indices. 

 The only material discrepancy is in the ray G ; and it is this ray for which Mr. Kel- 

 LAND himself has always found theory in excess in the calculation of Fraunhofer's 

 indices, and has made some remarks on the point in his memoir. Upon the whole, 

 considering this as the extreme case as yet known and examined, the superiority of 

 Mr. Kelland's method will be sufficiently manifest ; and it will be allowed that this 

 extreme case has been thus brought as far at least within the limits of accordance as 

 we can perhaps reasonably expect in the present state of our means of investigation. 



The case of sulphuret of carbon at the temperature of 12° is also brought into very 

 satisfactory agreement with theory by the present method. 



The other case of the same substance at the temperature of 22° still exhibits some 

 discordance. The ray G is here again in excess ; but the differences follow no re- 

 gular order, being sometimes in excess, sometimes in defect. This at least shows 

 that although the series is not rapidly convergent, in this case the addition of another 

 term would not remove the discordance. 



With regard to the error which is always found so marked in the ray G, Mr. Kel- 

 LAND in a letter to me, observed that in that ray it would seem reasonable to enter- 

 tain some suspicion as to the experimental data. Now there is one circumstance 

 which may corroborate such suspicion. The determinations of the values of X, as is 

 well known, were made from the interference-spectrum, in which the blue end, with 

 its dark lines, is most contracted. In the refraction-spectra, (and more so in the more 

 dispersed,) it is the most expanded : and the dark bands which in the lower cases 

 appear single, in the higher are resolved into several lines, in some instances sepa- 

 mted by very sensible intervals : and this difference must be still more marked in 

 comparing the highly dispersed spectra with that of interference. The ray G, in par- 



