OK. Miss Nancy linked to a shader that solved a different problem. That one is not prismatic, but it is refracting and passes light through instead of making a 100% shadow.

Meanwhile, willyb thinks it's not possible in Poser. Well it is.

It involves creating three different refract nodes, each with a different IOR. You take the red from one, the green from another, and the blue from the last and put them together.

The dispersion will not quite be equal to real life, but it will be much better than nothing.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)