326 Mr KELLAND, ON THE TRANSMISSION OF LIGHT 



they explain a multitude of various and complex phenomena, have 

 stamped them with a character so firm that it would be presumptuous 

 to attempt to set them aside. Truth however compels me to state, 

 that whilst I feel the highest admiration of M. Fresnel's theory, I am 

 at the same time doubtful whether some of the points on which it 

 rests are not defective, at least as commonly stated. I allude only 

 to the mechanical part of it; nothing can be more complete or more 

 elegant than the geometrical part. I trust I shall not be understood 

 in anything which follows as endeavouring in the slightest to detract 

 from M. Fresnel's fame. I mean far otherwise ; but having advanced 

 the opinion that some parts of the mechanical theory are inaccurate, 

 it becomes incumbent on me to explain in what manner this inaccuracy 

 is introduced, and how it happens that from imperfect premises accu- 

 rate conclusions have been deduced. 



It shall be my endeavour then to point out, as clearly as I am 

 able, the circumstances in which the theory labours under a difficulty, 

 and then to shew the cause of this difficulty. 



