Probably the oldest compass I have is an “engineer’s compass” similar to that in the photo below. These can be found at very modest prices so I suspect a number of readers might have one lying around.
I acquired mine a long time ago. So long ago, in fact, that it predated the internet and I had very little idea as to how to use it. Books and magazine articles on compass use concentrated on baseplate compasses such as the Silva and Suunto brands. What were all these wires and notches and tiny magnifiers for?
I had to work out how to use it myself. The method I came up with involved tilting both the cover and the lens bracket inwards so that they just made contact. I could look past the wire through the notch on the lens bracket. I could read the value off the dial using the lens. The gentleman in the photo below seems to have made the same conclusion.
Nowadays there are a wealth of websites and online manuals on how to use a lensatic compass. I wasn’t quite right, but I was not that far wrong either.
The lens bracket should be tilted inwards. By experimenting with the angle you can improve your image of the dial. According to most manuals the cover should be vertical for using the sighting wire. You will find, however, that if you incline the cover inwards it casts a shadow over the compass and the reflection of the wire allows you to read the value on the dial through the lens more accurately.The Books