. 84 W. B. Rogers on Binocular Vision. 
minated below by the angular point formed by the junction of 
the lower ends of a and d, which lines now constitute a V pat- 
allel to the plane of the paper. The whole figure is that of the 
three edges of a solid angle pointing downwards, formed by a, d, 
and the resultant 6 c, the two former ina plane parallel to the 
paper, and the latter inclining towards us from the apex. 
By a slight change of convergence we may next bring a and 
¢ to coincide, which is best effected by eae the attention to 
the two angular points of the figure. The same convergence 
will of course unite b and d, and the pestis figure will be a 
simple V lying ina plane parallel to the paper 
By converging the axes toa still remoter point we can next 
cause aanddto unite. In this case we obtain a figure composed 
of the perspective resultant of these two lines in a ‘position reced- 
ing as it ascends, and the lines 6 c united at their lower ends to 
form a V/ ina plane parallel to the paper. In other words we 
have a solid angle like that in the preceding case except that the 
perspective edge 7 beyond the plane of 6 ¢ instead of being on 
the — side of i 
n the ihe of the diagram diverge at a very smal] angle 
these cae follow in quick alternation, but the V or second 
figure above mentioned is that which most frequently recurs and 
is most polos: a mae due no doubt to the circumstance that 
all parts of th e formed by the same axial convergence: and 
hen the diagram contains a vertical line we can readily ad 
just it on the upper stage of the stereoscope so as to make this 
line visible at the same time to both eyes, and thus have a double 
of the line in alternating combinations with the rest. Ad- 
justing fig. 52 in this one we obtain the iieatteiens ves result 
52. 
ba 
a 
é¢ 
By stab the eyes alittle beyond the eae) m3 
of the paper, we may unite 5 with c, and d with c, y 
forming two perspective resultants each marked by 
the dot. These approach us as they extend up- 
wards. At the same time a and ¢ are brought nearer 
together and are seen to the left and right of the 
resultants severally. Indicating by the arrow heads 
the nearest extremities of the resultants, the entire ° 
effect of this combination is ee by fig. 53. 
4 
