with the visual line of the opposite eye, and therefore makes an 

 angle with its own visual line equal to the visual angle. But 

 this is not trae. Fig. 3 shows that it ought to make but half 

 that angle. If these figures therefore represent truly the posi- 

 tion of the images, as indeed they do, then they do not repre- 

 sent truly the visual or apparent positions of the visual lines. 

 The truth is, in natural vision the visual lines are shifted as well 

 as the images of all objects not situated at the point of sight, 

 and to the same degree, so that their positions relative to the 

 visual line are perfectly maintained in the visual result. 



Figures constructed on the usual plan give correctly the 

 position and distance of objects seen single, but fail to represent 

 truly the place of double images. They are well adapted to 

 express binocular combinations of similar objects or similar 

 figures on the plane of sight, as in my previous experiments; 

 and to some extent also in the stereoscope, but are unadapted 

 to express the results of binocular vision of natural objects. 



I propose therefore a new, and I am convinced, far truer mode 

 of representing the results of binocular vision, applicable to all 

 cases. I am satisfied that if this mode; had alw;i\.s been used 

 much confusion would have been avoiil.d. Souk- pieliminary 

 explanation will be required to make tlic luetluxl dcur. 



If a single object, as a finger, be placed tx-lore tlu> eves in the 

 median line of sight and the eyes be directed tu a distant point, 

 the object will be seen double, the heteronvmous images being 

 separated by a space exactly equal to th'e interocular space. 

 Now, the nose is no exception to this law ; the nose is always 

 seen double and bounding the cmnmon field of view on either 

 side. Again if two similar objects or figures be placed before 

 the eyes in the plane of sight and separated by a space equal 

 to the interocular distance, and the eyes be directed as before 

 to a distant point, both objects will be doubled, but two of the 

 doubled images, viz: the right eye image of the right object 

 and the left eye image of the left object, will combine to form 

 a single hijiocular image in the middle, while the right eve image 

 of the left object will be seen to the left and the left eye image 

 of the right object will be seen to the right. Thus there will 

 be three images seen ; the middle one binocular, the right one 

 belonging to the left eye alone and the left one belonging to the 

 nght eye alone. Now, the eyes tfiemselves are no exception to this 

 law. In binocular vision the eves themselves seem to double; 

 *^o of them combining to form a binocular eye in the middh 

 which looks out between the two noses, while the other two are 

 on either side bevond the noses. Each eye seems to itself to 

 occupy the central position while it sees (or would see if the 



