1864.] 



Human Eye in relation to Binocular Vision, 



The line will appear like a curved line, the convexity of which looks down- 

 ward. "When you look to its right extremity, it seems to rise from the left 

 to the right ; when you look to the left extremity of the line, the left end 

 seems to rise. In the sam.e way, all straight lines which go through the 

 peripheral parts of the field of vision appear to be curved, and to change 

 their position a little, if you look to their upper or their lower ends. 



This explanation relates only to Monocular vision ; we have to inquire 

 also how it influences Binocular vision. 



Each eye has its field of vision, on which the visible objects appear dis- 

 tributed like the objects of a picture, and the two fields with their images 

 seem to be superimposed. Those points of both fields of view which ap- 

 pear to be superimposed are called corresponding (or identical) points. 

 If we look at real objects, the accurate perception of the superimposition 

 of two different optical images is hindered by the perception of stereoscopic 

 form and depth ; and we unite indeed, as Mr. Wheatstone has shown, two 

 retinal images completely into the perception of one single body, without 

 being able to perceive the duplicity of the images, even if there are very sen- 

 sible differences of their form and dimensions. To avoid this, and to find 

 those points of both fields of view which correspond with each other, it is 

 necessary to use figures which cannot easily be united into one stereoscopic 

 projection. 



In fig. 2 you see such figures, the right of which is drawn with white 

 lines on a black ground, the left with black lines on a white ground. The 

 horizontal lines of both figures are parts of the same straight lines ; the 

 vertical lines are not perfectly vertical. The upper end of those of the 

 right figure is inclined to the right, that of the left figure to the left, by 

 about 1^ degree. 



Now I beg you to look alternately with the right and with the left eye 

 at these figures. You will find that the angles of the right figure appear 

 to the right eye equal to right angles, and those of the left figure so appear 

 to the left eye ; but the angles of the left figure appear to the right eye to 

 deviate much from a right angle, as also do those of the right figure to the 

 left eye. 



When you draw on paper a horizontal line, and another line crossing it 

 exactly at right angles, the right superior angle will appear to your right eye 

 too great, to your left eye too small ; the other angles show corresponding 

 deviations. To have an apparently right angle, you must make the vertical 

 line incline by an angle of about 1^ degree for it to appear really vertical ; 

 and we must distinguish, therefore, the really vertical lines and the appa- 

 rently vertical lines in our field of view. 



There are several other illusions of the same kind, which I omit becatise 

 they alter the images of both eyes in the same manner and have no influ* 

 ence upon binocular vision ; for example, vertical lines appear always of 

 greater length than horizontal lines having really the same length. 



q2 



