198 



On the Normal Motions of the Human Eye, [April l4, 



meridians, and which is nearly as great as the distance of the feet of 

 the observer from his eyes when he is standing. Therefore, when we look 

 straight forward to a point of the horizon, the Horopter is a horizontal plane 

 going through our feet — it is the ground upon which we are standing. 



Formerly physiologists believed that the Horopter was an infinitely 

 distant plane when we looked to an infinitely distant point. The differ- 

 ence of our present conclusion is consequent upon the difference between 

 the position of the really and apparently vertical meridians, which they did 

 not know. 



When we look, not to an infinitely distant horizon, but to any point of 

 the ground upon which we stand which is equally distant from both our 

 eyes, the Horopter is not a plane ; but the straight line which is one of its 

 parts coincides completely with the horizontal plane upon which we are 

 standing. 



The form and situation of the Horopter is of great practical importance 

 for the accuracy of our visual perceptions, as I have found. 



Take a straight wire— a knitting-needle for instance — and bend it a little 

 in its middle, so that its two halves form an angle of about four degrees. 

 Hold this wire with outstretched arm in a nearly perpendicular position 

 before you, so that both its halves are situated in the middle plane of your 

 head, and the wire appears to both your eyes nearly as a straight line. 

 In this position of the wire you can distinguish whether the angle of the 

 wire is turned towards your face or away from it, by binocular vision only, 

 as in stereoscopic diagrams ; and you will find that there is one direction 

 of the wire in which it coincides with the straight line of the Horopter, 

 where the inflexion of the wire is more evident than in other positions. 

 You can test if the wire really coincides with the Horopter, when you look 

 at a point a httle more or a little less distant than the wire. Then the wire 

 appears in double images, which are parallel when it is situated in the 

 Horopter line, and are not when the point is not so situated. 



Stick three long straight pins into two little wooden boards which can slide 

 one along the other ; two pins may be fastened in one of the boards, the third 

 pin in the second. Bring the boards into such a position that the pins are 

 all perpendicular and parallel to each other, and situated nearly in the 

 same plane. Hold them before your eyes and look at them, and strive to 

 recognize if they are really in the same plane, or if their series is bent 

 towards you or from you. You will find that you distinguish this by 

 binocular vision with the greatest degree of certainty and accuracy (and 

 indeed with an astonishing degree of accuracy) when the line of the three 

 pins coincides with the direction of the circle which is a part of the 

 Horopter. 



From these observations it follows that the forms and the distances of 

 those objects which are situated in, or very nearly in, the Horopter, are 

 perceived with a greater degree of accuracy than the same forms and 

 distances would be when not situated in the Horopter. If we apply this 



