The thingy I’m looking for is a simple tube, (can be made out of carton, plastic or metal) about the size of a finger. It contains some sort of prism, that allows the user to determinde the wavelenght of a certain color.
Oliver Sacks once said he carried one with him at all times because he wanted to understand more about color and light.
It’s called something like …scope. If anyone could tell me the name of the device, so I and can start hunting for one on E-bay, I would be most grateful.