W. B. Rogers on Binocular Vision. 85 
If now we remove the point of convergence a little further, so 
as to combine @ with c and e with c, we obtain from them two 
perspective resultants approaching us as they extend downwards, 
and since the same convergence serves to unite } with d, we 
have a third perspective resultant between the other two, dipping 
away from us. Under these conditions the result is that indica- 
ted by fig. 54, 
_ By converging the axes toa point still more distant we may | 
combine a with d, and e with 6, and as these pairs are severally 
parallel, the resultants will be in a plane parallel to the paper. 
At the same time the line c as seen by the right eye will appear 
on the left side of the resultant of ad, and as seen by the left 
eye will be placed on the right side of the resultant of be. The 
eflect will be that represented in fig. 55. 
Directing the eyes to a still more distant point we may next 
unite @ with e so as to form a perspective resultant in the middle 
of the optical picture. At the same time 8 and d will exchange 
sides, and ¢ being doubled will appear on the extreme right or 
Aes These transpositions and combinations are indicated in 
The four distinct combinations above described are observed 
to alternate with one another rapidly and in different ways with- 
out our being conscious of any effort in producing them, and 
hence the unpractised observer may find it difficult to retain 
either of them long enough for deliberate examination. A few 
trials, however, will enable him to do it with ease. 
PART FOURTH. 
e Of the coincidence of unequal figures. . 
_ It was first observed by Prof. Wheatstone that “two squares 
‘‘reircles differing obviously but not extravagantly in size” may 
through binocular combination with or without the stereoscope 
