What do these symbols mean in equations 4 and 5 on this page?

Question

I'm looking at an old math textbook on the Internet Archive. On page 17 of this pdf at the bottom it has 2 formulas.

One has what looks like an apostrophe and the other has a dot (like a decimal but floating).

Question: Can someone explain what these symbols mean?

Answer 1

It's just a comma and a period — mathematical text is still text.

Presumably you wouldn't be as surprised if you saw

... it follows that: $I_0=\dfrac{Pdi}{360}$, and $I_e=\dfrac{Pdi}{365}$.

but in order to emphasize and label the equations, we put them on their own line. We nevertheless should keep the punctuation. Take a look at this MathOverflow thread.