80 



3» 3d 3i 5« 3/ 3t) 



"jj" 17' 3y ""^' "^ ~" ^' 

 «, /3, y, being the cosines of the angles which the ray makes with the axes of x, y, z, and v 

 being the velocity, estimated on the material hypothesis, and considered, in extraordinary sys- 

 tems, as a homogeneous function, first dimension, of the cosines a, fi, y. Reason for calling 

 this principle the Principle of Constant Action ; analogous principle respecting the mo- 

 tion of a system of bodies. General consequences of this principle : generalisation of the pro- 

 perties of optical systems, considered in former sections of this essay ; the phenomena of coloured 

 systems depend on the partial differentials of the characteristic function, taken with respect to the 

 colour . . . . . 161 to the end. 



CONCLUSION. 



• Review of the chief results of this essay, and of the manner in which they may be useful ; and 

 remarks on the researches of former writers, respecting the properties of systems of rays. 



