What might measure the spirit of a viewer (5)
I believe the answer is:
optic
I believe this is a double definition.
'what might measure the spirit' is the first definition.
(an optic is used to measure alcoholic spirits)
'of a viewer' is the second definition.
(relating to eyes - things that view)
(Other definitions for optic that I've seen before include "Spirits dispenser" , "bar assistant" , "Describing a sensory organ" , "Device for measuring dispensed alcohol" , "Ocular" .)