330 W. B. Rogers on Binocular Vision. 
If however we produce the intersections in somewhat quick suc- 
cession, we: have a perception more or less distinct of the per- 
spectiveness -of ‘each of these intersecting lines, while a and ¢ 
tending to coalesce causes the former to lose its parallelism to 6, 
When the perspective effect is fully developed we perceive a and 
c only as they are united in the resultant. The axial movement 
has connected the ‘sense of perspectiveness with each of them 
causing each to be perceived in the binocular direction. 
ow it must be remembered that this perspective position and 
binocular direction being the result of successive combination of 
points in the two lines, is produced as well when the contiguous 
points thus united lie near the centre as when near the ends of the 
lines. But in the former case, that is, where the points about O 
in ¢ are united with those about O in a, the line & must form its 
image centrally in the right eye when a@ forms its image in like 
position in the left. In these circumstances therefore the retinal 
images of a and 6 fall on corresponding points, at the same time 
that the former line is perceived in the binocular perspective po- 
sition intersecting but not coinciding with the latter. In this 
momentary phasis of the experiment, it is obvious that the result 
is at variance with the law of corresponding points. When how- 
ever the parts of a and ¢ which the eyes successively combine 
are not near the centres of these lines, the image of 6 will be 
formed away from the retinal centre of the right eye, while that 
of the combining points of ¢ will be in that centre, and the im- 
age of a will be in the retinal centre of the left eye. In this case 
the law of corresponding points requires that a and 6 should ap- 
pear distinct from one another as they really do. Thus therefore 
at is only when the resultant perspective is Sormed b combining 
the central points of the component lines that this experiment con- 
tradicts the law in question. 
But the characteristic feature in this class of effects is not so 
much the departure in certain conditions from the theory of cor- 
responding points, as the general fact of which this is but a par- 
in the same eye. 
In explaining the experiment of Wheatsone, above referred to, 
Sir David Brewster, if I rightly understand his remarks, (Phil. 
Mag., June, 1844,) ascribes the disappearance of a as a vertical 
line to an actual invisibility of the vertical image, or part of it, 
caused by its proximity on the retina to the image of the other 
component line. Of the fitful vanishing and reappearing of pa 
of the figure long since noticed in such cases by this eminent ob- 
server, 1 am abundantly satisfied from my own experience ; but 
fg alate: 
