92 PSYCHOLOGY. 



at them oblige us, so long as tliej are not very distant, to 

 converge our eyes. Hence approacli of tlie thing is tlie proh- 

 able objective fact when we feel our eyes converging. Now in 

 most persons the internal recti muscles, to which converg- 

 ence 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. Mak^ 

 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 insufiicient, 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 with- 

 out changing its distance. Enlargement of retinal image 

 is therefore an ambiguous sign. An opera-glass enlarges 

 the moon. But most j)ersons will tell you that she looks 

 smaller through it, only a great deal nearer and brighter. 

 They read the enlargement as a sign of apj)roach ; and the 

 perception of apj)roach makes them actually reverse the 

 sensation which suggests it — by an exaggeration of our 

 habitual custom of making allowance of the apparent en- 

 largement 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 mag- 

 niiy 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, 



