Mystery Hoard has a nice profile of Thomas. Here's an excerpt:
Thomas produced her first reels for View-Master in 1946 --a series of Fairy Tales and Mother Goose rhymes that are still in circulation. According to one source, Thomas "developed special methods of close-up stereo photography and modeling which is now in common use by major motion picture studios" (John Waldsmith, Stereo Views, 1991). She created scenes of such detail and attractiveness that you feel you could step inside and look around a corner at a complete world. Besides the Fairy Tales, these worlds included versions of the Frankenstein and Dracula stories, scenes from the comic strip Peanuts, and 3-D versions of animated cartoons like The Flintstones. Amazingly, all of the puppet-like figures were sculpted from clay and the scenes were shot using a single-lens camera (not a stereo camera) that was moved on a track to get the stereo shot. Sometimes the models were moved slightly between shots to enhance the 3-D effect. During her heyday, Thomas appeared on television and radio to satisfy the curiosity of the children who consumed View-Masters by the millions in the 1950s and 60s. Today, she is largely forgotten except for a few collectors. You can see her at work (in 3-D, of course!) on this collector's reel. A tv appearance is available on this dvd.
Drawn! pointed me to some wonderful work by Brian Butler "who has not only scanned in some Flintstones ViewMaster slides, but has made animated GIFs transitioning between both the left and right stereoscopic images, creating a glasses-free approximation of the slides’ 3D effect." Check it out, it's really cool!