In Poser, a mirror is a specular reflector and this does not reflect light back into the scene. However, we can add an invisible diffuse reflector that would reflect light back into the scene. (As does every other object in the room that isn't a mirror.)

The 40 bounces seems darker because I did it with physical accuracy - each bounce is only 90% as bright as what arrived. It is impossible to achieve 100% reflection in the real world.

So the first bounce is 90%, the second is 90% * 90% = 81%, the third is 90% to the 3rd power = 72.9%. After 12 bounces it is 90% to the 12th power = 28.2%. After 40 bounces it is 90% to the 40th power = 1.48% of the original brightness.

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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)