Easily overlooked and sometimes mistaken for decorations, glass tiles embedded in the sidewalk are often indicators of subterranean rooms. Called "sidewalk prisms" or "vault lights," these pieces of glass are set in the sidewalk next to a building or in storefront entrances, in order to catch sunlight and cast it into a space below or deeper into the basement. Sunlight passes through the surface and then through a prism which angles the light inward, illuminating the space with natural light.
The concept behind vault lights originated with glass "deck lights" used on the decks of ships in order to illuminate their cargo hold without the use of candles or lanterns, reducing the risk of fire.
Starting in 1845, different systems of glass lenses embedded in concrete or steel were patented for use in sidewalks. They met with great success, and building codes began to recommend vault lights as a fireproof source of light for basements. By the 1880s, large firms such as the Luxfer Prism Co. were manufacturing vault light systems and marketing them as a way to increase a building's floor space (which in those days was limited by lighting) and improve working conditions by maximizing natural light. Vault lights emerged at a moment when enthusiasm for modern building technology was joined by a growing awareness of the role natural elements play in quality of life.
While never as common as in Chicago, Boston, New York, or Seattle, at one time it would not have been unusual to find vault lights in Toronto. The Canadian Luxfer Prism catalogue listed dozens of Toronto buildings that used their prismatic products, including the Lumsden Building at Adelaide and Yonge streets, the Ryrie Building at 299 Yonge, and the Heintzman Piano factory in the Junction (now demolished). However, their popularity was relatively short-lived. Installed by private property owners, many light wells spilled out onto public sidewalks. The City would not pay the extra cost involved in maintaining or repairing the prismatic glass, and soon neither would the building's owners. As the cost of electric lighting decreased, there was less incentive for them to invest in the maintenance and repair of their vault lights, and these prismatic glass light wells were slowly replaced by solid pavement. Vault lights have all but disappeared from Toronto's streets — the only known example is the glass in front of the shops at 2869 Dundas Street West (near Keele).