AND THE THEORY OF LIGHT. 51 



time to move from the place of percussion in concentric 

 spheres to great distances. And in like manner^ when a 

 ray of light falls upon the surface of any pellucid body, 

 and is there refracted or reflected, may not waves of vibra- 

 tions or tremors be thereby excited in the refracting or 

 reflecting medium at the point of incidence, and continue 

 to arise there, and to be propagated from thence as long- 

 as they continue to arise, and be propagated when they 

 are excited in the bottom of the eye by the pressure or 

 motion of the finger, or by the light which comes from the 

 coal of fire in the experiments above mentioned ? And are 

 not these vibrations propagated from the point of incidence 

 to great distances ? And do they not overtake the rays of 

 light, and by overtaking them successively do they not put 

 them into fits of easy reflexion and easy transmission 

 described above? For if the rays endeavour to recede 

 from the densest part of the vibration they may be alter- 

 nately accelerated and retarded by the vibrations over- 

 taking them" 



A moment^s reflection will show us that both of these 

 suppositions cannot be admitted into the same theory of 

 colour without rendering it extremely complex, not to say 

 incomprehensible. 



73. Sir John Herschel sums up a review of the two 

 theoi'ies in these words : " The fact is that neither the 

 corpuscular nor the undulatory, nor any other system 

 which has yet been devised will furnish that complete and 

 satisfactory explanation of all the phenomena of light 

 which is desirable. Certain admissions must be made at 

 every step as to modes of mechanical action, where we are 

 in total ignorance of the acting forces ; and we are called 

 on where reasoning fails us occasionally for an exercise of 

 faith." 



74. I have adduced these authorities to show that with 

 the admissions and corrections and exercises of faith which 



