You are looking for the slits, these show up in Akro game marbles or "solids" as well--just a tell that it is an Akro--the "fisheye" will flow around the entire marble as you backlight it while turning it--sometimes the fisheye will be limited to the slits—sometimes not—it depends upon the depth of the marble.
I wish that I could show you this in an image—not yet—I have tried—many times.
Marble—On!!