I was using Helmholtz coils to bend an electron beam into a circle. You can see the curve of the beam within the top of the Helmholtz coils. I wanted to measure the diameter of the beam but it was inside the glass bulb and there was no ruler scale inside the tube. So I put a sheet of glass outside the tube.
Then I set up a ruler and illuminated it with a lamp. There was a partial reflection in the glass so the ruler appeared to be inside the glass bulb. The great thing about the glass is that you can see through it as well as seeing reflections in it. This is the source of the Victorian music hall special effect called Pepper's Ghost. It is important that the ruler be the same distance in front of the glass as the glass is distant from the centre of the bulb. That's because a reflection is always the same distance behind the mirror as the object is in front of the mirror.
Here's a clearer picture of the electron beam, bent into a circular path by the uniform magnetic field.
