Double Refraction of Quartz. 207 



When the ambiguity takes place near the right or left hand 

 of the center, it is the ordinary ray only which is furnished. 

 When it takes place nearly above or below the center, it is the 

 extraordinary ray only which is furnished. 



The following conjecture may, perhaps, without impropriety, 

 be attached to this Paper. It is the suggestion of an explanation 

 of the unequal refrangibility of differently coloured rays. 



To account for the difference of refrangibility, we must sup- 

 pose that the velocity of waves of different lengths is different 

 either in air, or in the refracting medium, or in both. If it were 

 different in air, it would affect the aberration of stars by a quan- 

 tity that might be sensible: there is no reason to think that this 

 is true. It is probable therefore that the difference is wholly 

 within the refracting medium. Now it is particularly to be re- 

 marked, that the difference of velocity does not depend on the 

 magnitude of vibration of each particle, for it is the same, 

 whether the light be feeble or intense, that is, whether the vi- 

 bration be small or great. Nor does it depend on the relative 

 vibration of two contiguous particles, as that varies in the same 

 proportion as the last, with a variation of the intensity. The 

 only element which, in conjunction with either of these, will 

 define the undulation, is the time of vibration : and it is in fact 

 the time of vibration which distinguishes the different kinds of 

 light. It would seem natural therefore to seek for an explanation 

 of the difference of velocities in something which depends not 

 on space, but on time. Now we have every reason to think 

 that a part of the velocity of sound depends on this circumstance : 



