A question concerning FIg. 7.11 on page 173. It seems to me, in light of what is said on the preceding page about null surfaces, that he has interchanged "normal" and "tangent" in the third and fourth sentences of the figure caption. I would say: "The tangent to the surface l lies in the surface and along the generating light rays. A normal vector t is also shown." Anyone agree with this? It is not included in the (rather long) list of errata I downloaded. Thanks for you help or corrections.