Directing experiment to understand sensation as well as perception.
Take about 15 index cards as well as a flashlight that is opaque on all sides (therefore that light shines only through the front) into a very dark room. Afterward placing all 15 cards over the beam of light gradually remove the cards one at a time pending you can barely detect the light, and then count the number of cards that remain over the light. After a few minutes the light must begin to look brighter. When this is the case add a card as well as see if you can still see the light. Repeat this process of slowly adding cards over a 15-minute period. Were you able to notice an increasingly dim light the longer you expended in the dark?