XVII. Oti the Intensity of Light in the neighbourhood of a Caustic. 

 By George Biddell Airy, Esq. A.M., Astronomer Royal: 

 Late Fellow of Trinity College, and Plumian Professor of As- 

 tronomy and Experimental Philosophy in the University of 

 Cambridge. 



QRead May 2, 1836, and March 26, 1838.] 



^VHF.N a great physical theory has been established originally on 

 considerations and experiments of a simple kind, which by degrees 

 have been exchanged for comparisons of more distant results of the 

 theory with more complicated cases of experiment, it has always been 

 considered a matter of great interest, to trace out accurately by mathe- 

 matical process the consequences, according to that theory, of different 

 modifications of circumstances : which can then be compared with 

 measures that have been made, or that may easily be made in future. 

 It is with this view that I solicit the indulgence of the Society, for 

 the following investigation of the Intensity of Light in the neigh- 

 bourhood of a Caustic, as mathematically estimated from the Undu- 

 latory Theory. 



The investigation which I present here belongs, ostensibly, only to 

 the case of reflection. The introductory part of it will, however, (with 

 the proper modifications) apply equally well to all cases of refraction 

 and all combinations of reflection and refraction. There seems also to 

 be no reason why the latter part (the estimation of the intensity of 

 light, by considering the wave of light when it leaves the last surface 

 to be divided into a great number of small parts, whose separate effects 

 Vol. VI. Pakt III. sC 



