projection only and not a solid, continues to present the front view of an object which it represents wherever the spectator happens to stand.  Were the eye in the portrait a real eye, a side movement of the spectator would, it is evident, cause him to see less of the pupil and more of the side of the eyeball, and he would only continue to see the full pupil when the eye followed him.  We regard the eye in the picture as a real eye having relief, and judge accordingly.

We may fall into similar illusions respecting distance in auditory perception.  A change of wind, an unusual stillness in the air, is quite sufficient to produce the sense that sounding objects are nearer than they actually are.  The art of the ventriloquist manifestly aims at producing this kind of illusion.  By imitating the dull effect of a distant voice, he is able to excite in the minds of his audience a powerful conviction that the sounds proceed from a distant point.  There is little doubt that ventriloquism has played a conspicuous part in the arts of divination and magic.

Misconception of Local Arrangement.

Let us now pass to a class of illusions closely related to those having to do with distance, but involving some special kind of circumstance which powerfully suggests a particular arrangement in space.  One of the most striking examples of these is the erroneous localization of a quality in space, that is to say, the reference of it to an object nearer, or further off than the right one.  Thus, when we look through a piece of yellow glass at a dull, wintry landscape, we are disposed to imagine that we are looking at a sunny scene of preternatural warmth.  A moment’s reflection would tell us that the yellow tint, with which the objects appear to be suffused, comes from the presence of the glass; yet, in spite of this, the illusion persists with a curious force.  The explanation is, of course, that the circumstances are exceptional, that in a vast majority of cases the impression of colour belongs to the object and not to an intervening medium,[42] and that consequently we tend to ignore the glass, and to refer the colour to the objects themselves.

When, however, the fact of the existence of a coloured medium is distinctly present to the mind, we easily learn to allow for this, and to recognize one coloured surface correctly through a recognized medium.  Thus, we appear to ourselves to see the reflected images of the wall, etc., of a room, in a bright mahogany table, not suffused with a reddish yellow tint, as they actually are—­and may be seen to be by the simple device of looking at a small bit of the image through a tube, but in their ordinary colour.  We may be said to fall into illusion here in so far as we overlook the exact quality of the impression actually made on the eye.  This point will be touched on presently.  Here I am concerned to show that this habit of allowing for the coloured medium may, in its turn, occasionally lead to plain and palpable illusion.