Can you work out what this is?

This is an optical instrument dating back to the middle of the last century.  Bottom right you will see there is a focusable eyepiece.  At the other end is a knurled ring that adjusts the width of the slit visible in the top right picture.  Top left attempts to show what you can see through the instrument.  [NB this is not a kaleidoscope or indeed any sort of toy;  it came from the electron microscopy unit of a major London hospital.]

Can you work out what it is and what it might be used for?  I do know the answer to this one and will share it tomorrow.