2(38 Dr. G. J. Stoney on the Resolution of Light 



wavelets substituted for it, without producing any change in 

 nature. Each one of these u f w's (undulations of flat 

 wavelets) travels in the direction of some radius of the 



Fis. 1 



^ 





J??a??e 



Reference-hemisphere. 



reference-hemisphere, and has its wave-fronts parallel to 

 the tangent-plane at the outer end of that radius We may 

 call undulations outward-bound when they travel along the 

 radius from centre to surface of the hemisphere, and inward- 

 ly ou ad when they travel in the opposite direction. 



9. Practically, in most real problems, we know beforehand 

 whether we are dealing with outward-bound or inward-bound 

 undulations, so that no appreciable inconvenience results 

 from the circumstance that two undulations — one inward- 

 bound and the other outward-bound — are represented by the 

 same radius cPi- We may call cF l the guide-radius of 

 whichever of the two we have to deal with. It may equally 

 well be represented by the point P 1 on the hemisphere (which 

 we may call its guide-point) ; and still better by Ii, the 

 orthogonal projection of P 1 on either the base-plane or some 

 parallel plane (which we may call its index-jjoint). For 

 purposes of mathematical investigation it is more convenient 



