“* 
80 W. B. Rogers on Binocular Vision. 
Arr. X.—Observations on poe Seer. mere ; by Professor 
Wituiam B. Roce 
PART THIRD 
Of successive or alternating combinations of lines. 
Wuen the figures presented to the two eyes consist of lines _ 
capable of being united in two or more different ways these com- ~ 
binations may be produced successively by a voluntary change of 
convergence, and in certain cases they are observed to follow one 
another in quick eesnetian without our being conscious of 
effort in producing t | 
21. Alternation of te lines. ; 
The simplest example of this effect occurs when a figuell com- 
posed of three equal verticals is so placed in the stereoscope that 
we may unite one of the extreme lines with either of i 2 other 
lines successive 
Thus placing fig. 45 on the upper stage of the instradaben so 
rn that a may be in front of the left eye while 6 
: and ¢ are both seobaseen to the right eye, we may 
combine a with 6; and keeping the eyes directed 
to their resultant, we at the same time observe 
the line ca little to the right. Now changin th 
optic convergence to a point somewhat more 
mote we can cause a to quit b and to ie: r 
or less rapidly over to c leaving 6 alone on 13 
left. By thus changing the convergences back- 
wards and forwards we may continue to unite a alternately with 
band ¢ with but little effort and as often as we please. When. 
the distance between b and c is very small, say one twentieth of 
an inch, this change seems?to me almost involuntary and occu 
with the rapidity of a flash whenever we transfer our attention 
from the resultant ab to the simple line ¢, or back again from the 
resultant ac to 6. Indeed, as we view the resultant and the paral- 
lel beside it, the line @ is seen as it were to flit backwards and — 
forwards between b and ce, and it is only by fixing the attention 
resolutely on the resultant that we can prevent aA alternate — 
ecomposition and recomposition of the lines. A dot placed a 
little above the line a, by accompanying the line in its novela 
enables us to mark its sticcessive union with 5 and ec. i 
22. Alternation of Vertical with Oblique lines 
Still more curious illustrations of giiariating combinations are 
n the picture includes one or more inclined lines 
as in fig. 46. Placing this on the upper stage of the cathe . 
oo tha the left, and 4 and ¢ in front of 
Ye bc 
