PRISM GLASS is a commercial high quality glass for exterior and interior windows, with its face patterned in rows of prisms hat direct light to places where it is wanted. Thus the same principle of optics that makes possible the modern binocular field glass and range-finder is turned to the broadest practical account in the daylighting of factories, offices, and homes. Exactly how the principle applies is explained on page 137.

Prism glass does not produce light, for it can gather and direct only what light there is. But its practical effect is almost the same as if it actually did produce light, since it does increase to an astonishing extent the available light in the interior of buildings.

Thus, in rooms on a deep court or well, or on a narrow street bordered by tall buildings, all the light that strikes the prism sheet is concentrated where desired.

The saving in artificial lighting effected by prism glass is so well recognized that formulas for all purposes and situations have become highly exact. Knowing the relative height of obstructing buildings and their horizontal distance from a given window, a simple calculation tells exactly the kind of prism required.

All the Pittsburgh Plate Glass Company's Warehouses are in position to give expert advice and to deliver the particular pattern best adapted to any use, on large scale or small. Prism glass is one of the important structural materials of the industrial world, for it renders many an interior, otherwise too dark for any good use, profitably serviceable.

Pressed Prism Tiles are made in squares either four or five inches square. Tile prism work is set in hard metal, all lights being reinforced with steel bars to make them solid and rigid. Geometric designs made from sheet prism glass set