Interesting. I'm not sure I understand why he's concerned about the catchlight, though. Shouldn't he be focusing on how the light is falling on his subject's face--presuming he's shooting a portrait ? Perhaps he's using the marble as an easy way to get a rough guide about which way the greatest source of light is coming from and positioning accordingly. My concern would be: can't he just look at his subject to see how the light is falling on his subject's face ?