I just bought a used R H Burton pupilometer and have some questions about it since there was no user's manual with it. There is a little toggle switch directly under the opening that the optician looks through. (I'm not talking abut the occluder lever on the underside which allows for mono measurements.) It seems to shift two different lenses in front of my eye, but I can't tell any difference between them. What is it for?

Also, there are two buttons on the top side. One is marked MAIN, the on/off switch, but the other button is marked PD/LD. I have no idea what that means!

If anyone has a user's manual for this, I would gladly buy it from you.
Thank you!