When we direct the axes of the two eyes to any definite object, its different parts affect corresponding parts of each retina, and the object appears single. When we squint, or do not look direct at the object, its images affect non-corresponding parts of the two retinæ, and the object appears double. The more widely the axes of the eyes are deflected from the object, the further asunder the double objects seem; and the less the axes are deflected the less distant from each other the double objects appear. Thus, when we hold the finger in front of the eyes while we look at a distant candle or gas jet, the flame appears single and the finger double. When we turn the eyes to the finger it appears single and the flame appears double.