436 
Proceedings of the Royal Society 
left eye only is looking at a distant object, and accommodation may 
not be entirely suspended in tbe right eye, in spite of the assertion 
of Bonders that accommodative effort is always the same in each 
eye. This objection would be met by the fact, that when the two 
central images are in the same vertical line as in fig. 4, distant 
objects are seen in each which are really separated by an appreciable 
horizontal interval. It is difficult to represent natural objects 
diagrammatically in the holes ; but if we let them be repre- 
sented hypothetically by two vertical parallel lines at infinite dis- 
tance, one red and the other blue, the red line would appear in one 
hole and the blue one in the other, as in fig. 7. In this both eyes 
c 
© 
© 
d 
f 
Fig. 7. Different objects are seen in c and d. 
Blue. 1 Red. 
Two hypothetical lines at infinite 
distance. 
e c d f 
% ©© ® 
Fig. 8. The same object is seen in c and d. 
are fixing distant objects. Again, when the images of the holes 
appear to be separated, as in fig. 8, the same object may be 
seen in each. The distance between the holes in this case repre- 
sents nearly what would be the true interaxial distance when the 
axes are parallel (fig. 9). It is even possible for the two holes to 
continue separate for a little time when made exactly level, though 
usually they rush together without much delay. The simple ex- 
periment, with modification, is also available to determine the 
obliquity of the intercentral line, and the slight obliquities of the 
respective meridians of the two retinae with the eyes at rest. For 
this purpose I make the holes through a piece of cardboard. 
