13dde39e   
   From: mathma18@gmail.com   
      
   On May 21, 2:17 am, "J-C"  wrote:   
   >  wrote in message   
   ----   
   > As to Descartes ovals they are pretty useless in practice for a number of   
   > reasons, while they may be stigmatic they do not fulfill the sine condition   
   > for example, therefore they can't be used for imaging extended objects.   
      
   Thanks.(Working in India I have only a limited or ready access to such   
   good old book sources. Apart from internet, trying to  derive it   
   afresh).   
      
   But under what situations is Snell's law not valid ? Why cartesian   
   ovals, with optical length invariance considered,do not obey the law?   
   What situation makes it to re-conform ? ( I mean, when did the law   
   become illegal :) ?).   
      
   Narasimham   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)