154 Mn KELLAND, ON THE DISPERSION OF LIGHT, 



posing a refracting substance, as should lead to results in unison with 

 those of observation. There are, however, a variety of complex pheno- 

 mena, to w^hich scarcely any modification of Newton's hypothesis will 

 apply, whilst that of undulations accounts for them in the clearest and 

 most satisfactory manner. The phenomena of dispersion for a con- 

 siderable time stood almost alone in the way of this theory, and ap- 

 peared incompatible with its principles. It was assumed, and with 

 good reason, that colour was dependent on the lengths of a wave, 

 whilst the velocity of transmission determined the refractive index of 

 the medium. It became then evident that the theory was at fault, 

 unless the velocity of transmission within refracting media could be 

 shewn to depend on the length of a wave. AVhat was still worse, 

 from the appearance of the stars we were forced to aUow, that light 

 of all colours was transmitted uniformly through vacuum. 



Several suggestions Avere made, which, if they did not remove the 

 difficulty, tended at least to clear the theory from suspicion of inca- 

 pability, and to shift the ground of attack from the principles them- 

 selves to our power in applying them. Thus IMr Airy, reasoning from 

 analogy, observes : " We have every reason to think that a part of the 

 velocity of sound depends on the circumstance that the law of elasti- 

 city of the air is altered by the instantaneous developement of latent 

 heat on compression, or the contrary effect on expansion. Now if this 

 heat required time for its developement, the quantity of heat developed 

 would depend on the time during which the particles remained in 

 nearly the same relative state; that is, on the time of vibration. Con- 

 sequently the law of elasticity would be different for different times 

 of vibration, or for different lengths of waves : and therefore the ve- 

 locity of transmission would be different for waves of different lengths. 

 If we suppose some cause, which is put in action by the vibration of 

 the particles, to affect in a similar manner the elasticity of the me- 

 dium of light, and if we conclude the degree of developement of that 

 cause to depend on time, we shall have a sufficient explanation of the 

 unequal refrangibility of differently coloured rays." 



These observations are important, inasmuch as they remove from 

 the Undulatory Theory the imputation of being inadequate to account 



