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| : 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 



