Takes me back to organic chemistry classes decades ago where molecules were often printed in stereo pairs to break out of 2D thinking about 3D structures. One could use special glasses or train the eyes to see them. The latter method had the risk of getting left/right reversed which is a big deal when looking at chiral molecules where "handedness" is critical.

I have had a burning question about these since forever. Why do e.g. the Magic Eye books use divergent, not convergent (i.e. crossed-eyes) types? I find it much easier to control position and focal length when crossing my eyes. I wonder about others' experiences.

I had never heard of SIRDS before. That's a very cool way to potentially show stereograms to folks that struggle to see them!