180 W. B. Rogers on Binocular Vision. 
greatly vary throughout these lines, while between a and c they 
much greater for the lower than the upper halves. The 
same remarks apply to fig. 81, except gi: 
that in this case it often happens that 
unite simultaneously or nearly so with 
the corresponding parts of c. In all these 
cases of course the union does si extend 
to the extreme parts of the lin 
n consequence of the fa scatilon of optical convergence 
whether spontaneous or voluntary, the various modes of union 
above described present themselves in irregular alternation, but 
the setae of these combinations, that of 6 with the whole ‘a ¢, 
is the one which most frequently occurs first and which con- 
tinnes reset 
he experiment of Prof. Wheatstone referred to above, in- 
cludes, it will be seen, thé same conditions as that last described. 
It was as follows. He presented the letters (S) and (A) drawn 
of equal height and enclosed in equal circles, one to each eye. 
On attempting to combine them, he says, “the common bor- 
der will remain constant while the letter within it will change 
Sipe from that which would be perceived by the right ea 
at which would be perceived by the left eye alone. At the 
vba of change the letter which has just been seen breaks 
into fragments while fragments of the letter which is about to 
appear mingle with them and are immediately after replaced by 
the entire letter.” (Phil. a -, April, 1852. 
a b c 
ribed. In these circumstances most of the A disappears in 
the resultant and sometimes, especially when"the eyes are fa- 
tigued, the whole letter seems for a moment to vanish. The 
“Upper or lower part of the heavy stroke of the oe and sometimes 
_ the whole of this part of the letter are thrown into perspective 
and | pene eee in or and ayitude, but ‘f ae very rarely 
observ: to ppe 
As i the ie a of the letters in Prof. W.’s figure, and 
1é rapid changes of combination thence resulting, I have f found 
‘difficult to mark the phenomena — I prefer using let- 
