Well yes - that would be 26inches by seven eighths of an inch.

The first number is the diameter of the wheel you are using (and hence the diameter the tyre needs to be) and the second number is nominally referring to the width of the tyre. So at 7/8 of an inch you are looking at about 22mm.

So your 650x23 tube would probably be just fine. FWIW the ones I buy are usually bracketted - 18-22mm or 23mm-25mm rather than just one size.