W. B. Rogers on Binocular Vision. 213 
Owing to the small inclination of the lines in figure 23, a very 
slight fluctuation of convergence is sufficient to effect the suc- 
cessive union of many corresponding points, and this taking place 
involuntarily and almost instantaneously, gives us the perspective 
_ single or double resultant as the first or natural binocular effect. 
The effort necessary to prevent this slight and spontaneous fluc- 
tuation of convergence is such that unpractised observers find it 
difficult to bring the intersecting lines into the parallel plane. On 
first view they are apt to regard the result as a single perspective 
line, and it is only through a close and more deliberate inspection 
that they discover the two nearly coalescing intersecting lines of 
which the perspective figure consists. It is however, easy after 
some experience in these experiments, to maintain suc rfect 
fixity of convergence at different stages of the binocular combi- 
nation, as to give the resultant the form of intersecting lines in 
the parallel plane. 
similar gradation of effects is presented when the figures just 
referred to are placed on the lower stage of the stereoscope in the 
same order as before, but with this general difference that each 
corresponds in result to lines of less inclination on the upper 
Stage. ‘Thus I find that fig. 20 combined by cross vision in front 
of the lower stage, presents appearances like those above de- 
scribed in fig. 21; and so fig. 21 gives the perspective lines proper 
to fig. 22 on the upper stage, while this latter on the lower stage 
gives the almost single perspective line of 23 in the upper. 
this comparison the upper stage is supposed to be 15 inches and 
the lower 31 inches from the rear end of the instrument. 
This different grade of effect with the same figures, according as 
the lines are combined in front or behind their actual position, is 
evidently dependent on the much greater range of axial rotation 
reqttired in the former than in the latter case in order to combine 
successively given parts or the whole of the inclined lines. ‘Thus 
in fig. 24, if L and R denote the centres of the left and right 
eyes respectively, a d the lower and 6c the upper ends of the 
lines placed on the upper stage, anda’ d’ and W/ ¢ the same points 
of the figure as placed on the lower stage, it is obvious that aand 
d will be binocularly united at s and 6 and ¢ at r; also that a’ 
