>> Assuming your sphere is to the "right" of your ray starting point.
>
> No assumption.
>
> The result of this equation is the distance along the ray that you need
> to travel to get to the intersection. Therefore, the lowest solution is
> guaranteed to be the one you want. (Unless the camera is /inside/ the
> sphere, which won't work for several other reasons besides this one.)

No assumption, except for this one, that one, and these other ones, too...

:P

-- 
/*Francois Labreque*/#local a=x+y;#local b=x+a;#local c=a+b;#macro P(F//
/*    flabreque    */L)polygon{5,F,F+z,L+z,L,F pigment{rgb 9}}#end union
/*        @        */{P(0,a)P(a,b)P(b,c)P(2*a,2*b)P(2*b,b+c)P(b+c,<2,3>)
/*   gmail.com     */}camera{orthographic location<6,1.25,-6>look_at a }

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