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rf ROGER, Mae VO aay PRE a EE eer Spt 
W. B. Rogers on Binocular Visioh 333 
In this experiment the resultant appears as nearly single as in ‘ 
the case of two slightly inclined lines (p. 21260 that unless we 
pause to fix the gaze on one of the ends,we cannot ‘observe a 
“want of coincidence at the other extremity. The fhoment how- 
same time the resultant subsides from its perspective position. 
When the coincidence of the two lines is such as to present 
the perspective effect, I cannot observe the disappearance of one 
or other extremity as mentioned by Sir D. Brewster. Asa test © 
of this I place a small dot on the outer edge of the perforations 
at A and band I find that these marks always remain visible 
while the resultant is seen in perspective. I may add that such 
a disappearance would not explain the seeming coincidence of 
the unequal lines in a perspective resultant, but that on the con- 
trary the formation of this resultant, as in the case of mutually 
inclined lines requires the actual though not simultaneous union 
of the corresponding ends. 
The effects above described are readily obtained by using lines 
marked at the ends by short cross lines or by dots placed below 
or above them, and effecting the combination either without a 
stereoscope or by placing the figure on the upper or lower stage. 
42. 
i i j L j 
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ee 
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Fig. 42 represents the proportions of the lines which I have found 
convenient for either of these methods of operating. With this 
I obtain at once or very promptly, the binocular coincidence and 
the perspective effect. The dotted lines below denote the perspec- 
tive directions, the arrow heads marking the nearer ends of the re- 
sultants. The upper of these indicates the resultant when the lines 
are united by using the upper stage or by looking beyond the pa- 
r, the lower when they are united by the lower stage, or ata 
plane nearer than the paper. : 
When the lines are not, as in the preceding experiments, 
strongly marked at the ends, the effect of the combination is va- 
iable, presenting sometimes a partial superposition of one line or 
the other without relief and sometimes a perspective resultant 
I find however that by using broad lines strongly contrasting with 
the paper, I can in general obtain the latter result quite promptly, 
but the effect is more fluctuating and difficult to maintain than 
when the ends are conspicuously marked. 
It is to observed that in all these experiments the perspective 
effect is directly dependant on the successive union of the cor- 
“ee 
