>    weight_norm is not the length of the ray. It comes straight out of
>    Kaczmarz' method. weight_norm is the denominator in the first (and only)
>    display equation on the Wikipedia page:
>    [1]http://en.wikipedia.org/wiki/Kaczmarz_method

>    If you want a better explanation, Kak & Slaney give a brief derivation of
>    Kaczmarz in chapter 7.

Thanks for the explanation!  In fact, I got confused because of this part
of the docstring "A measure of how long the ray's path through the
reconstruction circle was": it made me think that this parameter is a
length. Maybe the docstring could be modified to avoid such confusion?