oe 
its extremity is marked by the dot and that 
W. B. Rogers on Binocular Vision. 81 
right eye, we may alternately combine a@ with 6 46. 
and with c, giving rise in the former case toa 
perspective resultant (ab) which recedes as it de- 
-Scends, and in the latter case to one (ac) which 
approaches as it descends. By fixing the atten- 
tion on either resultant fora moment we see that 
: mee . 6¢ 
therefore it includes the line a along with one of 7 
the others. If now we turn to the remaining line we observe 
the dot and with it the line @ to flit over to this line and we are 
” ble 
18 no doubt due to the fact that when the combination is made at 
the angle, the lower point of a is at the same instant united with 
: the lower end of both 6 and. ec, and that as the change of converg- 
ence necessary to combine the adjoining lower part of a wih 
that of 6 andc¢ successively is extremely slight, the ay Dae 
_ descending perspective lines near the angle are formed in an 
change of convergence is required in passing from the one com- 
bination to the other, and therefore a much longer time is con- 
sumed in forming the successive resultants. 
It has been remarked above that in looking at one of the result- 
ants the other line soon after subsides from its perspective posi- 
tion. It may naturally be asked why this line does not lose its 
®ppearance of relief the moment it ceases to be a resultant, that 
s, the moment the line a is observed to quit it. The explanation 
Szooxp Sanus, Vol. XXI, No. 61.—Jan, 1856. Log 5 yomily 
Fi, 
