The DECstation 5000/25 machines (familiarly called Maxines) have monitors
that include rather complicated controls.  If the monitor becomes
distorted, or it becomes difficult to read, you can reset the monitor to
its original state.  To do so, look for a small indented hole on the right
side of the monitor, just below a rotating dial, and press the button
inside.  The button is depressed, so you will need to insert a pin or
something skinny to get to it.

If you want to know more about how the controls work, there is a brief
manual for the monitor available for perusal in the OLC office (1st
floor N42).