Before electric lighting became common around 1900, light was provided free of charge by the sun, and at night by candle, lamp, or other flame. Anything which could extend the reach of the sun's free and safe light to interior spaces would make that space more useful and valuable. Prism glass is architectural glass used to redirect sunlight to interior spaces through refraction and reflection.
|Left: Martin Place; Right: Mark Foys|
|Left: George St portico of GPO; Right: diagram from Glassian|
In Sydney, I have identified installations from:
B C Plummer $ Co., Sydney
Leopold Barnett & Co. Ltd
Hayward Brothers, UK
St Pancras Ironwork Co., London
Go see for yourself. They are scattered up the eastern side of George St, along the northern side of Martin Place between George & Pitt, along Mark Foys Elizabeth Street frontage, and along the northern side of Oxford Street. They are probably elsewhere, too. Their use declined as electric light became cheaper and better, and by the 1930s were on their way out. Now, they are endangered relics.
I am indebted to The Glassian site for the technical text of this post.