164 Mk KELLAND, on the DISPERSION OF LIGHT, 



observe that in the sequel, owing to the negative value of one of the 

 terms (I) there adopted, it is clear from experiment that the above 

 form is incorrect. 



It is true, some subsequent hypothesis might be ne.cessary to adapt 

 the formula as we have it to all cases, but for the present we have a 

 form as simple as possibly can be obtained, and whose interpretation 

 will be a matter of little difficulty. Before, however, I proceed to 

 such interpretation, it may be useful to examine how it applies to the 

 known dispersions of a number of glasses and other substances, since, 

 unless it has some pretensions to supply us with results coinciding 

 with those of observation, it can have little claim on our notice. 



SECTION II. 



Examination and Illustration of the Formuki. 



Let \ represent the length of a wave ; v the velocity of transmission. 



Now from the equation 



a = « cos {nt-kp) we deduce 



a = a cos \nt— ihp + -^j \, 



or the same state recurs after intervals of -y-, which is consequently 

 the length of a wave, or X. 

 Also we obtain 



a = «COS {w(< + A^)-^(/3 + Ap)], 



wherever w.A^=A.Ajo; 



