What everybody else says. For a really good summary of all this, check out Sheldon Brown's article:Tire Sizing Systems

Probably, the 26 x 1 3/8 is 590mm 650A for EA3 rims. Unless it's Schwinn, but they were usually marked as 26 X 1 3/8 X 1 1/4 or something.

Somewhere on the tire, there should be some other printing to help you figure out exactly what it is -- probably says 590 somewhere.

26 x 1 1/2 could very well be a 650B, 584 mm bsd tire. Somewhere on those tires it should say something: either 584 or 559.

Things are not getting better with the tire designations. Some people are now calling 584 mm tires 27 1/2. Perhaps they think if you take the old size (26 1 1/2) and sum it up (27 1/2) it makes sense.

edited to fix typo.