Vintage measuring device - What am I?

This arrived in a box of jewellery from a car boot sale. It is a device that works like a spirit level when all three prongs are pressed level the dial reads 0.
Any idea what it’s called please?
Nicely made and heavy just marked British Made and Patent.
Thankyou

 

eyeglass radian apparatus - found it via a photo search app, but how the heck you use it is beyond me at the moment :

https://www.google.com/search?q=eye...mg&ei=8hwtYdHfL82DtQXyj6roBg&bih=523&biw=1280

 

this tells a bit about what it does: https://www.framedisplays.com/meraap.html

 

+6.00 on front of lens -6.00 on back=0 no prescription
+6.00 on front of lens -4.00 on back=+2.00 i.e reading glasses

 

It doesn't have enough numbers to measure my milkbottle lenses...

 

You are clever to find that!. My search terms were woefully inadequate. Thankyou x

 

Lol