SENSE OP DIRECTION :—SINGLE VISION. 433 
inverted, and that this error is only corrected by experience. 
The cases alluded-to in the last paragraph, however, satisfac¬ 
torily prove this assumption to be incorrect ; since, although 
the individuals saw everything upon the same plane, as in a 
picture, the representation was erect from the first. The fact 
now appears certainly to be, that we have an intuitive sense 
of direction , which guides us in our appreciation of the actual 
situations of objects and parts of objects; so that, when a visual 
impression is made upon any part of the retina, we see the 
point from which the rays proceed, in the direction of a line 
drawn from the affected spot of the retina through the common 
centre (fig. 210, h) through which all the rays pass, this line 
serving as a true guide to the actual place of the object. 
55 9. The same may be said of the cause of single vision , 
that is, of our seeing but one object, although its picture is 
double , being formed upon both retinae. In animals which, 
like Man, look straight forwards, the field of vision of the 
two eyes is nearly the same; so that most of the objects seen 
with one eye will be seen with the other also. The objects 
at the right and left sides of the field of vision, however, are 
seen with the right and left eyes singly ; yet we perceive no 
difference in the aspect of these from that of the objects 
towards which both our eyes are directed. It is evident, 
then, that the pictures formed on the two retinae are blended, 
as it were, by the mind, into a single perception. This seems 
to be, in part at least, the effect of habit. When the images 
do not fall upon parts of the two retinae which are accustomed 
to act together, double vision is the result. Thus if, when 
looking steadily at an object, we press one of the eyeballs 
sideways with the finger, the object is seen double. In the 
same manner, if an affection of the nerves or muscles of one 
eye (as happens temporarily in intoxication) prevent it from 
being directed to the same point with its fellow, double vision 
of all objects is the result. This, when it does not soon pass 
away, is a not unfrequent symptom of serious disorder within 
the brain. If it continue long enough, however, the indivi¬ 
dual becomes accustomed to the double images, or rather 
ceases to perceive that they are double, probably because the 
mind becomes habituated to receive them, or rather perceives 
only one of the impressions on the two parts of the retinae 
which now act together. For if, after the double vision has 
