Floors near the elevators on the third floor of the U.S. Capitol, just outside the Senate Daily Press Gallery, are set with glass panels. Below is a glass ceiling. The panels are called laylights and allowed light to spread in the days before electric lighting. (John Kelly/For The Washington Post)

What’s the story behind the glass panels in the floor outside the Senate chamber in the U.S. Capitol?

Elliot Carter, Washington

You may be familiar with a line in the Bible: “And God said, ‘Let there be light.’ And there was light.”

Easy for you, God. Humans have it a little tougher.

Glass panels, or laylights, allowed light to spread in the days before electric lighting. (John Kelly/For The Washington Post)

These days, it’s easy to take light for granted. Illumination is available at the flick of a switch. This was not always the case, especially when natural sunlight was all we had at our disposal.

Lighting a small structure — a one-room, single-level house, say — is fairly straightforward. Just put a window in each wall. Want to admit a lot of light? Leave a hole in the roof and cover it with transparent glass. Voila: skylight!

But what if the building has many rooms and several floors? What if the building is the U.S. Capitol?

Imagine, if you will, that it is the year 1893 and you are a Library of Congress clerk sent to fetch a tome from a shelf in the furthest reaches of the great book depository. You do not work in the handsome Jefferson Building — that won’t open for four years — but in the aforementioned U.S. Capitol.

When it was first built, its various rooms were lit with candles and whale oil lamps. In the 1830s, kerosene — a byproduct of petroleum also known as paraffin — illuminated the Capitol. A decade later, natural gas produced light.

But natural gas is not portable. And so, as you hunt for that book, you use some old technology. As the Washington Evening Star reported in 1893:

“The searchers for the books are compelled to use common brass kerosene lamps whenever they go into the alcoves or dark corners for books that are wanted by readers. These lamps are of the character that might set fire to the library at any moment. The vaults below in the crypt and the galleries under the dome are filled with the most valuable records . . . and at no time does a ray of light reach them save from matches or gas jets.”

The elevators on the third floor of the U.S. Capitol, just outside the Senate Daily Press Gallery, are shown. The floors near the elevators are set with glass panels. (John Kelly/For The Washington Post)

To wring every available lumen from whatever the light source — be it the sun or a glowing gas jet — architects can place a hole in the floor and cover it with glass, as was done on the third floor of the Capitol, between the elevator banks near the Senate Daily Press Gallery.

“Those are laylights, which allowed the transmission of light from upper floors to lower ones before the introduction of electrical lighting,” wrote Erin Courtney of the Architect of the Capitol’s office in an email to Answer Man.

A laylight can be in the ceiling — the ceilings in the Senate and House chambers originally had glass laylights — or the floor. Of course, one room’s ceiling can be another room’s floor.

Laylights can diffuse light as well as spread it. The National Gallery of Art has laylights to prevent glare and shadows on the artwork. Memorial Continental Hall on 17th Street NW, home to the library of the Daughters of the American Revolution, features an impressive laylight: 25 glass frames, each 8-by-9 feet, set in decorative metalwork.

The DAR’s laylight underwent an extensive nine-month renovation completed in 2013. The Christman Company’s Greta Wilhelm worked on that project. Answer Man wondered: How is a laylight different from a skylight?

“Basically, the skylight separates the exterior of the building and the interior of the building,” Greta said. “Skylights typically will have a pitch to them so they’ll shed water, whereas the laylight is horizontal and interior to the building. It could be beneath a skylight or just separating two interior spaces of the building and allowing light to transmit between them.”

The Capitol started transitioning away from gas light in the 1880s and by 1898 had converted almost completely to electric lights. The Star predicted that bright electric light would “prevent the acts of miscreants who are fond of congregating in the galleries during the night sessions and are shielded by the half gloom that is caused by the concentration of light in the center.”

Never forget: Democracy dies in darkness.

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/johnkelly.