Brightness

The tensile strength
of the string in the bow
limits the power available
to the arrow's impact.

The strength of the string
in the lyre, determines
the quality its sound.

The quality of the light
in the lantern depends upon
the brightness of the candle.

The knowledge available
in the pictured scene
depends on the luminosity of
of the filament in the bulb.

I.e.,
brightness is to knowledge
as strength is to power.

____________
I.e.,
and the chains are so strong and so bindin'
in this circle of hell
that the heart of man is broken.