In nature, nearly everything seen is light on, reflected light. The list of things that emit light — light through — is very short: the sun (and other stars), lightning, fire, bioluminescence and molten lava. Only fire and the stars can be stared at for any length of time. Before people learned to produce fire, it too would not necessarily be common. The flash of lightning is evanescent. Bioluminescence is faint and rare. For most, experience with molten lava would be unknown or unusual.
After the mastering of metal craft, glowing red hot objects might often be encountered. The transformation of metal from ore to tool or weapon was seen as a sacred rite.
Edison’s introduction of electric light was unsettling:
When the first electric lights cast their golden glow over Menlo Park on New Year’s Eve 1880, a crowd of 3,000 people gathered in awe. Edison, the worker of miracles, had triumphed. Historian and author Carolyn Marvin says, “Victorians saw the electric light and the effect of electricity (or ‘the lightning’) as having an almost religious power. Edison was both godlike, because he could manipulate the lightning, and a very dark and satanic figure for the same reason. He could challenge God’s order.”
For those whose only experience with illumination had been candle, kerosene or gas, Edison’s electric torchlight parade had men lit up like candles or lamps.
My speculation is that the mind evolved to process reflected light and that glowing and flickering
sources tend to open the doors of perception — as shown by the Dream Machine of Brion Gysin and William Burroughs. It follows then that reading on a screen is a sort of ricercar. The text is something secondary; the effect of the light source itself is the theme.