92 On the Negative or Protecti?ig Rays of the Sun. 



the corresponding point on the edge. A similar remark, as 

 respects impressions on paper, has been previously made by 

 Sir J. Herschel. 



Such are the chief facts I have observed in relation to the 

 Daguerreotype spectrum. It would seem at first sight that 

 their diversity is so great that we can have but little hope of 

 I'educing 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 prin- 

 ciple 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 move- 

 ment 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 lengtiis of the 

 luminous waves have the extraordinary relation of 2 : I. Then, 

 when we also see that, before a perfect neutralization of action 

 between two rays ensues, those rays must be adjusted in in- 

 tensity to each other, does it not show that interference of 

 some kind is going on ? Again, the yellow ray is in number- 

 less instances the ray which most completely antagonizes those 

 at the red and violet extremes of the spectrum : to use the 

 language of Sir J. Herschel, " this ray may be considered as 

 marking a sort of chemical centre, a point of ecjuilibrium, or 

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

 seeing that these pheenomena 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 feeble 

 violet one, and prevent it from decomposing iodide of silver, 

 merely because their relation of length is in the proportion of 

 li to 1, it should follow on the i-ame principles, that a red 

 ray acting conjointly with a violet should give rise to an in- 

 creased effect, because the lengths have now become 2:1. 

 And that this in reality is the case I found by direct experi- 

 ment ; for on projecting the red upon the violet, so that the 

 colours should half overlap each other, I found that at the 

 point of concourse the plate instantly solarized, and assumed 

 a splendid green metallic colour. 



I have now explained the acceptation in which I receive the 

 term negative ray as a synonym (in this instance of iodide of 

 silver) for the yellow ray, and alluded to the mechanism which 

 seems to be the cause of protecting action generally. Per- 

 haps on a review of his own experiments M. Becquerel may 

 find reason to believe that there are in reality antagonizing 



