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Some answered, that they did not remove the book to lessen the light, but to receive more rays, and let all the space between the letters and their eyes be filled with [p. 223] lightsome air. Others agreed with those that imagine the rays of vision mix with one another; for since there is a cone stretched between each eye and the object, whose point is in the eye and whose basis is the object, it is probable that for some way each cone extends apart and by itself; but, when the distance increases, they mix and make but one common light; and therefore every object appears single and not two, though it is seen by both eyes at once; for the conjunction of the cones makes these two appearances but one. These things supposed, when old men hold the letters near to their eyes, the cones not being joined, but each apart and by itself, their sight is weak; but when they remove it farther, the two lights being mingled and increased, they see better, as a man with both hands can hold that for which either singly is too weak.
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