W. B. Rogers on Binocular Vision. 209 
axes must be successively conveyed on every point of it. In 
the case of objects having relief, and in that of the perspective 
figures of the stereoscope, I am convinced that this process is, to 
a certain extent, necessary. But the observations above cited 
show that the principle of successive vision does not apply in the 
case of parallel resultant lines in a plane at right angles to the 
direction of view, or in that of actual lines or points situated in 
that plane; it, being understood in both cases that the objects 
observer appears to admit the latter view; for, in his beautiful 
observations on the binocular combination of the series of plane 
figures upon carpets and paper-hangings, he says: “these figures 
being always at equal distances from each other and almost per- 
fectly equal and similar, the coalescence of any pair of them, by 
directing the optic axes to a point between the paper-hangings 
and the eye, is accompanied by the coalescence of every other 
or”? 
18. 
The conditions of this simultaneous binocular union are illus- 
trated in (fig. 18), where @..bandc...d represent the two pairs. 
“ equi-distant points, or parallels. Here a and c are united at 7, 
the point to which the optic axes are supposed to be conveyed, 
and at the same time 6 and d are combined at s. Asa bandcd 
rs hence r and s, the two resultants, are practically at equal 
‘stances from a d, or from the line joining the centres of the 
two eyes. 
10. Conditions of vision of a physical line in a vertical 
Position, : 
When a straight wire, or other physical line, is held in a vertical 
Position before the ey2s, the visual conditions under which it is 
Stconp Srnies, Vol. XX, No. 50—Sept., 1855, 
