When writing on the blackboard, the size of letters or the color of chalk depend on that writer's point of view. However, this should be suitable for size and lightness of the room and eyesight of the learners. Everybody seems to know things like this by his experience, but still the exact information has not been obtained yet. It has been measured, then, the least recognizable size of letters on the basis of colors of the chalk, lighting on the blackboard, distance between the blackboard and the observing point, and eyesight of the learners. In blackboard itself and the brightness or the color of letters drawn with chalk have been also measured by the brightness spot meter. These data should be useful when writing on the blackboard in the class.
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