merit or demerit), the direction which actually exists in the other direction (for a point which is to the north of me is to the south of another point) is apprehended by itself, apart from the other elements of direction; the apprehension which actually takes place is thus likewise true. Similar is the case of the double moon. Here, either through pressure of the finger upon the eye, or owing to some abnormal affection of the eye, the visual rays are divided (split), and the double, mutually independent apparatus of vision thus originating, becomes the cause of a double apprehension of the moon. One apparatus apprehends the moon in her proper place; the other which moves somewhat obliquely, apprehends at first a place close by the moon, and then the moon herself, which thus appears somewhat removed from her proper place. Although, therefore, what is apprehended is the one moon distinguished by connection with two places at the same time--an apprehension due to the double apparatus of vision--yet, owing to the difference of apprehensions, there is a difference in the character of the object apprehended, and an absence of the apprehension of unity, and thus a double moon presents itself to perception. That the second spot is viewed as qualifying the moon, is due to the circumstance that the apprehension of that spot, and that of the moon which is not apprehended in her proper place, are simultaneous. Now here the doubleness of the apparatus is real, and hence the apprehension of the moon distinguished by connexion with two places is real also, and owing to this doubleness of apprehension, the doubleness of aspect of the object apprehended, i.e. the moon, is likewise real. That there is only one moon constituting the true object of the double apprehension, this is a matter for which ocular perception by itself does not suffice, and hence what is actually seen is a double moon. That, although the two eyes together constitute one visual apparatus only, the visual rays being divided through some defect of the eyes, give rise to a double apparatus--this we infer from the effect actually observed. When that defect is removed there takes place only one apprehension of the moon as connected with her proper place, and thus the idea of one moon only arises. It is at the same time quite clear how the defect of the eye gives rise to a double visual apparatus, the latter to a double apprehension, and the latter again to a doubleness of the object of apprehension.