You really need to get close enough to the knife edge so that the entire surface of the mirror is illumunated. This can be a problem but it is not to difficult to solve. I have a similar problem with a very small blind spot in my right eye so I've always built my testers to use my left eye. The basic design that Tex uses is really rather antiquated and can be improved very easily. This improvement also guarentees that the knife edge and slit are always lined up accurately. It also allows the use of a very small and cool light source, a small flashlight bulb with built in lens. I've been using this version for over thirty years with excellent results. If you are interested in details, e-mail me directly to and I'll send drawings. The only change to the Tex version is that the lamp housing is completely done away with so that it is very easy to use either eye. Another solution is to place a small very low power telescope behind the knife edge so that the objective is almost in contact with the knife edge. The shadows are them examined through the telescope. This has the advantage of much better eye relief and less dependence on the eye position when testing.

Jarvis Krumbein,