_Other illusions are due to the feeling of convergence_ being wrongly interpreted. When we converge our eyeballs we perceive an _approximation_ of whatever thing we may be looking at. Whatever things do approach whilst we look at them oblige us, so long as they are not very distant, to converge our eyes. Hence approach of the thing is the _probable_ objective fact when we feel our eyes converging. Now in most persons the internal recti muscles, to which convergence is due, are weaker than the others; and the entirely passive position of the eyeballs, the position which they assume when covered and looking at nothing in particular, is either that of parallelism or of slight divergence. Make a person look with both eyes at some near object, and then screen the object from _one_ of his eyes by a card or book. The chances are that you will see the eye thus screened turn just a little outwards. Remove the screen, and you will now see it turn in as it catches sight of the object again. The other eye meanwhile keeps as it was at first. To most persons, accordingly, all objects seem to _come nearer_ when, after looking at them with one eye, both eyes are used; and they seem to _recede_ during the opposite change. With persons whose external recti muscles are insufficient, the illusions may be of the contrary kind. * * * * * _The size of the retinal image_ is a fruitful source of illusions. Normally, the retinal image grows larger as the object draws near. But the sensation yielded by this enlargement is also given by any object which really grows in size without changing its distance. Enlargement of retinal image is therefore an ambiguous sign. An opera-glass enlarges the moon. But most persons will tell you that she looks smaller through it, only a great deal nearer and brighter. They read the enlargement as a sign of approach; and the perception of approach makes them actually reverse the sensation which suggests it--by an exaggeration of our habitual custom of making allowance of the apparent enlargement of whatever object approaches us, and reducing it in imagination to its natural size. Similarly, in the theatre the glass brings the stage near, but hardly seems to magnify the people on it. The well-known increased _apparent size of the moon on the horizon_ is a result of association and probability. It is seen through vaporous air, and looks dimmer and duskier than when it rides on high; and it is seen over fields, trees, hedges, streams, and the like, which break up the intervening space and make us the better realize the latter's extent. Both these causes make the moon seem more distant from us when it is low; and as its visual angle grows no less, we deem that it must be a larger body, and we so perceive it. It looks particularly enormous when it comes up directly behind some well-known large object, as a house or tree, distant enough to subtend an angle no larger than that of the moon itself.[96]