Thanks for the explanations. Here's a mystery (to me), though: In Bryce I made a lense using a boolean interection of spheres. Then I assigned a high-refractive-index glass material to the resultant lense. Then I positioned a round surface behind the lense. Half that "rear" surface could be viewed through the lense, whereas the other portion could be viewed to one side of the lense--unobscured. The lense distorts that portion of the image that is viewed through the lense. I now realize that Bryce doesn't bend the light. How then does it distort the image as though the light from the image is being bent? By the way, the lense distortion is "proportional" to its proximity to the interior edge of te lense, just as it would be with a real lense.