140 EXPERIMENTS AND INVESTIGATIONS 



equation to the curve M N in fig. 11 is y x = a, a conic hyper- 

 bola, and that the disposing force is inversely as the distance 

 at which the flexion of the rays bent and disposed takes 

 place. 



Scholium. It is clear that the extraordinary property 

 we have now been examining has no connexion with the 

 different breadths of the pencils at different distances from 

 the point of the first flexion, owing to the divergence caused 

 by that flexion. 



By the same kind of analysis, which we shall use in 

 demonstrating the 6th Proposition, it may be shown, first, 

 that the divergence of the rays alone would give a different 

 result, the fringes made by an inflexion following a deflexion 

 and those made by a deflexion following an inflexion; 

 secondly, that in no case would the equation to the disposing 

 force be the conic hyperbola, even where that fringe de- 

 creased with the increase of the distance ; thirdly, even where 

 the effect of increasing the distance is such as the dispersion 

 would lead to expect, the rate of decrease of the fringes is 

 very much greater in fact than that calculation would lead to, 

 five or six times as great in many cases; and lastly, that 

 instead of the law of decrease being uniform, it would, if 

 caused by the dispersion, vary at different distances from the 

 two edges.* Nothing therefore can be more manifest than 

 that the phenomena in question depend upon a peculiar pro- 

 perty of the rays, which makes them change in their dis- 

 position with the length of the space through which they 

 have travelled. 



It should seem that light may be compared, when bent and 

 thereby disposed, to a body in its nascent state, which, as we 

 find by constant experience, has properties different from 

 those which it has afterwards ; and I have therefore con- 

 trived some experiments for the purpose of ascertaining 

 whether or not light at the moment of its production (by 



* I have given demonstrations of these propositions in a memoir pre- 

 sented to the National Institute, but I am reluctant to load the present 

 paper with them. 



