Among my favorite educational maps and charts are those that are "slated" or have a blackboard surface. They are often referred to as chalk plates. These were designed to be written on in chalk by the instructor and wiped clean with a damp cloth. The graphics were usually very simple and clear- often just an outline. The simplicity of these maps and charts can be very artful and interesting. Chalk plates are relatively rare and difficult to find. Many are worn and cracked from being written on and wiped off countless times in the classroom.
Pictured below are a few examples from my collection:
But, this clock reminded me of some interesting color blindness charts that I will be selling at the flea. They were produced by an inventive optometrist Israel Dvorine in the 1940s to test aviators and recruits during WWII.Until this time, there were similar charts made in Japan to test colorblindness but they were difficult to obtain and reproduce. If you would like to read more about Dvorine and his charts you can click here for a lengthy and technically challenging article.
In the same vein, I have some fascinating stereoscopic charts by Dvorine for testing other vision irregularities.