The cardboard frame design was patented (Patent #1,500,025) by Alvin L. Mayer of Long Island, New York, and boasted the advantages of lower weight and reduced manufacturing cost. One disadvantage of Mayer's design was that by eliminating the protective pane of clear glass, the slide's image-carrying emulsion was exposed to scratches, dirt, and damage.
Of course, for a product originally designed to be discarded after use, this was probably not considered to be much of a downside.
|Slide for Coquette (1929) with cardboard shield in place|
Until the other day I had never seen an example of a slide with the cardboard shield still attached. Fortunately, a friend/collector shared with me a slide of of Mary Pickford's Cocquette (1929) manufactured by Combined Photo Industries, Inc., which still included the the protective shield. Now I know it's hard to get too excited about a 2 7/8" by 2 3/8" cardboard square, but having never actually a slide with the shield in place I thought it was pretty cool.
Finally, as a post script for the technically minded of you, here's the diagram from the original patent application. I have used yellow to highlight references to the protective shield (labeled A2 in the diagram).
|Diagram for Lantern Slide patent #1,500,025 (granted July 1, 1924)|