(Edit for typo's)

100 years ago I made this one in Bryce. Each mag glass is made of two glasseous sphere-parts, so each mag glass requires 4 bounces to have the light pass through (in/out front part in/out back part). 

So to have a light ray pass through all five looking glasses, max raytrace bounce should be at least 20. For this image, it was set slightly less, and as a result the last looking glass is not transparent any more - when looked through via the front one. That's causing the black pit midleft.

In general: a light ray is allowed to bounce around, it gradually dies along the way, but when it's still alive and the limit is met, it gets killed anyway. When you don't have that amount of bounces in your scene, rendertime is not going to suffer. In the other way around: when your render times boosts with lower values, then that amount of bounces was used indeed.

Which does not mean that the result is visible, you might have an object in the scene that has a lot of reflections between parts, which hardly show in the final image on screen. Then it pays off to lower the limits. And during test renders. I needed that when making this one (Mirror in Mirror with shiny cars):

http://www.main.artbeeweb.nl/?p=4003

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though