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IOWA ACADEMY OF SCIENCE 
selenium crystals, at 1, figure 21, a slit, at to expose only a 
narrow strip of an illuminated page and a lens (at 5) for focus- 
sing this illuminated portion on the series of crystals. The back- 
ground at 6’' corresponds to any limited background that the 
human eye might observe, the lens corresponds to the crystalline 
lens of the human eye and the crystals conventionally shown cor- 
respond to the rods and cones of the eye. 
Continuing the analogy to the eye, the electric current through 
the crystals bears certain resemblance to the nervous impulse go- 
