82 W. B. Rogers on Binocular Vision. 
is I think to be found in the disposition of the mind to retain the 
perception of relief previously associated with the line until im- 
ena with some other definite idea as to the distance of its several 
ar’ 
When for some time we continue to carry the eyes back wards 
and forwards along the upper et of the perspective resultant, a 
curious effect presents itself. ‘The other line after subsiding to 
the plane of the diagram and thus losing its previous relief may 
ie observed to continue this revolving motion notil it has taken 
rspective position the reverse of what it had before, in which 
gate’ its different points are at the same distance from the eye | 
as the corresponding parts of the resultant. This effect is in 
conformity with the law formerly mentioned (2) that while look- | 
looking intently on any one o nject we are in me ned to refer all | 
others seen at the same time to the same dista | 
If for b and c we substitute three or more =e slightly diver- 
gent lines as in fig. 48, the successive nnion o 
a with all of them is so rapid especially when 
the attention is directed to the angular point, as 
to cause them all to appear in relief seemingly — 
at the same time. Buta more steady gaze di 
rected to the upper part of the fissure will show 
the dot of a fitting from one to the _ as 
the line a forms the snecessive resulta é 
An interesting modification of this ronperitien is seen in ithe 
combination of a vertical with two intersecting lines equally and 
oppositely inclined to the vertical as in fig. 49. In this case the 
resultant presents the appearance of two intersecting 8% 
perspective lines. Here the effect is not produced 
by combining a with the whole of b and the whole 
a 
fices to unite the lower end of a with that of 8, 
and as this is true for each pair of corresponding 
points in the upper half of c and the lower of 4, it 
is plain that the same gradation of axial movement which com 
bines the lower half of 6 with that of a will simultaneously 
combine the upper half of c with that of a. It is only neces-— 
sary to continue the movement in the same direction, to combine _ 
the lower half of ¢ and the nae of 6 with the corresponding 
parts of a. Thus the two tive intersecting lines are 
produced by the combination aie ay and alternately of a 
with the two near halves and the two remote halves of # and ¢. _ 
At the moment when the two near halves are thrown into per- 
spective by uniting with @ the dots appear at their extremities, 
and when by a slight optical change the two remote halves are 
combined with a the dots are seen to dart across to the ends of 
these halves. 
