You see above an advertisement from the American Luxfer Prism Company that offers a way to bring daylight into buildings. Prisms can be installed horizontally with vault (or pavement or sidewalk) lights, floorlights, and skylights; vertically with bands of prisms in shop fronts; and at an angle, with prism canopies.
These devices, similar to the older deck prisms on ships, were popular during the late nineteenth and early twentieth centuries. The prisms provided a solution to the problems of shops and workplaces (and houses with gloomy dining rooms) in the long, narrow spaces typical of desirable city blocks, at a time when gas and electricity were competing as lighting sources. Business owners objected to the cost of artificial lighting, to its effects on merchandise (soot, difficulty of judging colors) and employees (eyestrain that caused clerical errors, lack of healthful sunlight and consequent low spirits). Daylighting offered an economical and seemingly permanent solution to these problems.
Below is the cover of a fascinating Daylight Prism Company brochure, which is full of photographs of installations in Philadelphia. You can enjoy it on the Internet Archive. To the right is one of the testimonials in the brochure.
That testimonial was not illustrated. But here is an advertisement for another firm that occupied the building. An enlarged detail has been added.
The window treatment is hard to interpret. But here is a more detailed 1960 photograph of the building, slightly lightened to show the prism installation above the main window.
The good news about 924 Arch is that, though the prisms are gone, the handsome building remains, at the southwest corner of Arch and Hutchinson. In addition, it may be that one of the illustrated installations, at the Dobson Mills in Falls of Schuylkill, survived as a feature of the Dobson Mills Apartments complex (see figures 6 and 7 in the gallery of the apartments’ Web site).
These images of lost buildings seem to show prism installations.
This post is a bit of a sideshow, designed to lead you to a page with a longer treatment of the subject (link below), one that includes vault lights (and their bit part in Bell, Book and Candle) and another, more sensational, use of glass, the blue glass cure or blue glass mania, which began with General A. J. Pleasonton of Philadelphia. That is the delusion of the title.