334 M» F. August on a new Species 



aion in the eye, the image in an eye in motion must appear 

 different from that in an eye at rest, while on the other hand 

 the image of a real object must be the same in an eye in rapid 

 motion as in one at rest. As now the virtual image of the 

 point of reflexion moving along the curve is precisely identical 

 as to form with the curve itself, it is impossible that the eye 

 can make any sensible movements. Or to express the same 

 thing in other words, no movement of the eyes would bring two 

 virtual images on to corresponding retina-points, which do not 

 already lie on such parts, since their images are necessarily con- 

 fined to those points of the retina on which they are excited. 

 The experiment therefore compels us to regard Brucke's theory 

 of binocular vision as untenable, especially since it has already 

 been rendered doubtful by Dove's objection. 



7. It may be asked how the experiment can be reconciled 

 with the well-supported doctrine of identical retina-points. 

 With reference to this point the observations made in paragraph 

 5 may be of some assistance. If the curves are too dissimilar 

 they afford no stereoscopic image ; that is to say, if the images 

 fall on parts of the retinae too remote, our imagination is unable 

 to unite them. This is precisely the same case as when we 

 regard an object whose dimensions are too great in proportion 

 to its distance from the eyes ; in that case only a small part of it 

 is seen stereoscopically, all the rest appears double. The identical 

 retina-points would thus, practically, in regard to solid vision, 

 only have the negative signification that two particular impres- 

 sions can be united into one when that in the one eye is not too 

 far removed from the place that corresponds to the position of 

 the impression in the other eye. If this condition be fulfilled, 

 the solid image appears at the intersection of the two rays, which 

 may be considered as drawn through the middle point of each eye 

 and the points of the retinas affected ; if these do not cut (if they 

 are not in the same plane), it is impossible, as is well known, to 

 perceive a single image. There are therefore two conditions 

 necessary for the formation of a single solid image. 



The limits within which the point of the retina impressed may 

 vary from the true corresponding point is probably partly 

 dependent on the will, but may be extended or reduced by 

 practice. This at least seems to be the explanation of the fact 

 that those accustomed to the clear observation of a point see 

 more easily all surrounding objects double, while those who 

 accustom themselves to stereoscopic vision can unite very remote 

 images into one. The principal difficulty in the way of the 

 experimental determination of these relations would arise from 

 subjective differences. 



How far this view is admissible others must decide. It is 



