Thanks for the tip. The Lynx R 15 and SCT Micro Trac 600 were nowhere to be found.
Im have been using the Kensington Orbit Optical model for several years and reviews are all over the place. Had I seen them 4 years ago, it would not have been my choice. Fortunately, I was oblivious to the negatives and am very happy with it. Yes, the ball is not fixed - as the Logitech - and yes, it fell out in the most embarrassing place, our public library, the only place - barring Central Africa - where DSL has either been outlawed or Verizon thinks that hicks in the boonies are sooo sixteenth century... The clank, even on the carpeted floor, had the librarian almost run for the door before she typed in the library's protected wi-fi access address on my laptop whose touchpad I banished as useless.
Since you're so well up on all things electronic, could you tell me if the Logitech Marble Mouse T-CM14 could be used with a Mac AND PC Windows 7 (as the Kensington - flawless on both) since it's now decommissioned off my old Gateway. It even came with a PS2/USB converter and I hate to junk it. The Logitech driver page doesn't show it but that could be because of its age. It would be used on either iMac or laptop PC USB ports. I hate the Apple rodent because of its need for a large cage floor.
BTW: The Kensington can be made into a scroll mouse, albeit by sacrificing one of the buttons. Thus I have no "context" feature but that hasn't been a problem. I guess every trackball two-button mouse would present such a problem - yes?