492 HISTORY OF OPTICS. 



ferent points of the phenomena, (a datum which it 

 is very difficult to obtain with accuracy by experi- 

 ment;) and partly on misconceptions of the theory; 

 and I believe there are none of them which would 

 now be insisted on. 



We may mention, also, another difficulty, which 

 it was the habit of the opponents of the theory to 

 urge as a reproach against it, long after it had been 

 satisfactorily explained : I mean the half-undulation 

 which Young and Fresnel had found it necessary, in 

 some cases, to assume as gained or lost by one of 

 the rays. Though they and their followers could 

 not analyze the mechanism of reflection with suffi- 

 cient exactness to trace out all the circumstances, 

 it was not difficult to see, upon Fresnel's principles, 

 that reflection from the interior and exterior surface 

 of glass must be of opposite kinds, which might be 

 expressed by supposing one of these rays to lose 

 half an undulation. And thus there came into view 

 a justification of the step which had originally been 

 taken upon empirical grounds alone. 



10. Dispersion, on the Undulatory Theory. A 

 difficulty of another kind occasioned a more serious 

 and protracted embarrassment to the cultivators of 

 this theory. This was the apparent impossibility of 

 accounting, on the theory, for the prismatic dis- 

 persion of colour. For it had been shown by 

 Newton that the amount of refraction is different 

 for every colour; and the amount of refraction 

 depends on the velocity with which light is pro- 



