The California Academy of Sciences has a room for naturalists.

The California Academy of Sciences has a room for naturalists who use various tools for collections and research, and can interact with each other, including a device for naturalists (who have not learned to paint) to copy specimens before the advent of cameras.
The key is a 45-degree semi-lens, which reflects the object in front and projects the shadow into the eyes. At the same time, it can see the paper below through the semi-transparent mirror and draw OK along the shadow.
Fig. 2 is my skull of alligator. Fig. 3 is the effect seen from above. Fig. 4 is a schematic diagram California Academy of Sciences