It is a mistake to suppose that sharp sight is accompanied by clear visual memory.  I have not a few instances in which the independence of the two faculties is emphatically commented on; and I have at least one clear case where great interest in outlines and accurate appreciation of straightness, squareness, and the like, is unaccompanied by the power of visualising.  Neither does the faculty go with dreaming.  I have cases where it is powerful, and at the same time where dreams are rare and faint or altogether absent.  One friend tells me that his dreams have not the hundredth part of the vigour of his waking fancies.

The visualising and the identifying powers are by no means necessarily combined.  A distinguished writer on meta-physical topics assures me that he is exceptionally quick at recognising a face that he has seen before, but that he cannot call up a mental image of any face with clearness.

Some persons have the power of combining in a single perception more than can be seen at any one moment by the two eyes.  It is needless to insist on the fact that all who have two eyes see stereoscopically, and therefore somewhat round a corner.  Children, who can focus their eyes on very near objects, must be able to comprise in a single mental image much more than a half of any small object they are examining.  Animals such as hares, whose eyes are set more on the side of the head than ours, must be able to perceive at one and the same instant more of a panorama than we can.  I find that a few persons can, by what they often describe as a kind of touch-sight, visualise at the same moment all round the image of a solid body.  Many can do so nearly, but not altogether round that of a terrestrial globe.  An eminent mineralogist assures me that he is able to imagine simultaneously all the sides of a crystal with which he is familiar.  I may be allowed to quote a curious faculty of my own in respect to this.  It is exercised only occasionally and in dreams, or rather in nightmares, but under those circumstances I am perfectly conscious of embracing an entire sphere in a single perception.  It appears to lie within my mental eyeball, and to be viewed centripetally.

This power of comprehension is practically attained in many cases by indirect methods.  It is a common feat to take in the whole surroundings of an imagined room with such a rapid mental sweep as to leave some doubt whether it has not been viewed simultaneously.  Some persons have the habit of viewing objects as though they were partly transparent; thus, if they so dispose a globe in their imagination as to see both its north and south poles at the same time, they will not be able to see its equatorial parts.  They can also perceive all the rooms of an imaginary house by a single mental glance, the walls and floors being as if made of glass.  A fourth class of persons have the habit of recalling scenes, not from the point of view whence they were observed, but from a distance, and they visualise their own selves as actors on the mental stage.  By one or other of these ways, the power of seeing the whole of an object, and not merely one aspect of it, is possessed by many persons.