he A W. B. Rogers on Binocular Vidion. cw Sal 
me direction at the same time and with equal velocity, it would 
seem to be a natural consequence that the feeling of perspective- 
ness should connect itself equally with both these lines when ob- 
served in any momentary phases of intersection. According to 
: the resultant lines (O for example) along these two lines in the 
‘s 
‘idea, depending on the greater or less rapidity of the converg- 
ing movement of the eyes. 4 
16. Conditions of the binocular vision of a physical line in a, 
perspective position. 
In describing under a former head (13) the appearances of a 
physical line or wire placed so as to coincide with the resultant 
of two inclined lines, particular mention was made of the perspec- 
tiveness of the two intersecting images of the wire as well as o 
the component lines of the resultant. A simple means of study- 
ing the former effect is obtained by fastening the point of a pén- 
knife into the end of a slender rod of wood (one of Faber’s pen- 
cils) at right angles to its length, and holding the rod in a steeply 
sloping position between the eyes and a sheet of paper placed on 
the table. When the rod has been adjusted so as to be distinctly 
seen by each eye, as for example in the position CD (fig. 27), the 
eyes being at R and L, if we converge the axes steadily upon © 
or D or any intermediate point as O, we see the rod double, the 
But if we glance along the rod without making a continued 
pause at any point of its length, the two images will be seen to 
ave a perspective position, continually varying in amount, at the 
same time that their mutual inclination and the point of their in- 
tersection changes. So long as the rod is held in this steep posi- 
tion in regard to the plane of the paper, the two images hever ap- 
proach so closely as to appear even for a moment to coalesce, and 
when we incline the rod at a much lessangle, although this tran- 
sient appearance of union often occurs, the general effect is that 
of two nearly coincident intersecting perspective lines. 
Instrument for the binocular analysis of a perspective line. 
The following simple apparatus (fig. 28), suggested by the 
above experiment shows in a very striking manner the variable 
the former of clear glass, the latter of thin board covered with 
white paper and each about five inches wide by ten high. These 
Srconp Serres, Vol. XX, No. 60.—Noy., 1855. 41 
