Small glass prism in brass old box, closes and is engraved with initials H.E. Any ideas?

I found this gadget in a charity shop, It is has been made of high quality metal, about 1 inch or 2.5 centimetres wide, it is nicely hinged and it contains a glass or Cristal prism, it also has a handle and internally a small armature.

Does anyone have any idea what this is and is what it is used for?

Thank you.