2so Dr. Wollaston on semi-decussation 
manifest cause why infants first begin to give the corres- 
ponding direction to their eyes, and we clearly gain a step in 
the solution, if not a full explanation, of the long agitated 
question of single vision with two eyes. 
It may perhaps to some persons appear surprising, that so 
many as three instances of a disorder which they presume 
to be rare, should have been witnessed by one individual ; 
but I apprehend, on the contrary, this half-blindness to be 
far more common than is generally supposed ; and I might 
with as much reason express surprise at its having so far 
escaped notice,* were I not aware how many facts commonly 
remain disregarded, merely for want of explanation. It is 
evident that I once, and for a long time, overlooked the in- 
ference that is to be drawn from this affection ; and if the 
disorder had not happened to me a second time, I might never 
have reconsidered its cause. 
Even since the preceding pages were written, I have met 
with two more cases of this disease. One of my friends has 
been habitually subject to it for sixteen or seventeen years, 
whenever his stomach is in any considerable degree deranged. 
In him the blindness has been invariably to his right of the 
* Richter, in the third volume of his Elements of Surgery, has a chapter on 
half-blindness, and part of it relates to what he terms amaurosis dimidiata. From 
one instance there given, he seems to have seen some cases similar to those I have 
described ; but he has not noticed the corresponding affection of the two eyes, or 
considered the sympathy between them. 
Anfangs-griinde Der Wundartzeneykunst. 
Vol. 3, Chap. 1 6, p. 478, 
