This exhibit is the active part of a Helium-Neon gas laser. Melles Griot were well known for making professional laser equipment.
The power supply would produce arount 11,000 volts of-load and this would initiate the discharge. The tube current when active was around 3 mA and the ballast in the HT lead and the power supply characteristics regulated the voltage to around 2kV depending on the tube.
This is the main reflecting mirror at the anode end.
The transmission window and negative connection.
The laser action takes place in the capillary tube with the gas reserve in the rest of the envelope.
An example of this type of tube in operation.
An end on view of the cathode end and mirror.
Diagram this type of Helium-Neon laser. The highly reflecting mirror will have a loss of, typically, 0.1%. The transmission window will pass around 1.0% of the light through.
Thanks to Robert Coleman for the diagram, operational photograph and PDF.
The wide glass tube envelope is 35 mm in diameter, and including the base is 225 mm tall.