If it is a very slow leak, you can just top it up yourself. There is usually a screw cap of some sort on the bottom. The trick is to find the right replacement fluid - something I've not been success at for my current compass. I have seen white spirit suggested for small top ups, but yours looks pretty major.
+1 for Robert White for compass service. I think they're a Ritchie dealer.
It is common for compasses to lose a little fluid, particularly if not protected from freezing. That one's lost a lot of fluid, so take it to the doctor. Something's wrong.