Diascope

The diascope test pattern reference for automatic setup is a transparency (Figure 3), built into all TK47B lenses.

When it is used to project a slide then at serves as a diascope.

The diascope passes light through the two-dimensional object (such as a photographic transparency, slide, or film) and uses a converging projection lens to form an enlarged image on a distant screen.

The diascope (a flat piece of glass) was used to press out the blood and expose the apple-jelly points of marginal growth.