94 INTERFERENCE OF RADIATIONS. [MEMOIR V. 



the daguerreotype spectrum. It would seem at first 

 sight that their diversity is so great that we can have 

 but little hope of reducing them to* a common system of 

 results originating in the same cause. I have, however, 

 been long led to believe that the explanation is to be 

 met with in the great and fertile principle of interference. 

 From this point of view I regard the action of rays of 

 every kind as being essentially positive, and that action 

 mainly consists in impressing a vibratory movement on 

 the atoms of the decomposing substance. It is to my 

 mind a fact of no common significance, that in those Vir- 

 ginia specimens the places of maximum protection in the 

 less and more refrangible regions fall where the lengths 

 of the luminous waves have the extraordinary relation 

 of 2 : 1. Then, when we also see that, before a perfect 

 neutralization of action between two rays ensues, those 

 rays must be adjusted in intensity to each other, does it 

 not show that interference of some kind is going on ? 

 Again, the yellow ray is in numerous instances the ray 

 which most completely antagonizes those at the red and 

 violet extremes of the spectrum : to use the language of 

 Herschel, "This ray may be considered as marking a 

 sort of chemical centre, a point of equilibrium, or rather 

 a change of action in the spectrum." I cannot avoid see- 

 ing that these phenomena are connected with the remark- 

 able fact that the waves of red, yellow, and violet light 

 are of lengths which correspond to 2, 1^, 1. 



If, then, a powerful yellow ray can hold in check a fee- 

 ble violet one, and prevent it from decomposing iodide 

 of silver merely because their relation of length is in 

 the ratio of 1-J : 1, it should follow on the same princi- 

 ples that a red ray acting conjointly with a violet should 

 give rise to an increased effect, because the lengths have 

 now become 2 : 1. And that this is in reality the case I 

 found by direct experiment; for on projecting the red 



