The Gas and Electric period started around 1885
after the invention of the electric light bulb.
Previous to this period all light sources pointed
up as they always involved a burning flame.
With the light bulb came the task light where
light pointed down and illuminated surfaces better.
The Gas and electric form where one light source is
pointing up and another pointing down really reminds
one of the adage "Necessity is the mother of invention".
The reason the gas and electric was combined on the same
fixture was because the new technology, electricity, was
not trusted. Thus you had the Gas and Electric form.