W. B. Rogers on Binocular Vision. 93 
Drawing chords from middle to the ends of } and combining 
the figure, thus modified, with a, we obtain, along with the per- 
_ Spective curve before seen, two perspective chords extending from 
_ its apex to the upper and lower ends. All these lines are devel- 
oped with perfect distinctness and so rapidly as to appear quite 
Simultaneous. By fixing the attention however on the apex of 
the perspective curve we see the right line separating from it at 
the ends, and by great steadiness of convergence we may even 
Succeed in bringing the vertical into the position of a tangent at 
the apex, in which case the figure loses its relief 
It will be found in the sequel that the perspective curve formed 
i by the union of a right line with the arc of a circle is in all cases 
\ @ conic section, the special nature of which is dependant on the 
conditions of the experiment. 
experiment the parts a 96 
When 4 and d are 
brought together we have a similar curve with its apex averted. 
| But in both eases the coincidence entirely fails towards the upper 
and lower parts of the figure. By passing rapidly from the one 
to the other combination, we have momentary glimpses: of a 
Warped surface which combining the two flexures appears convex 
©n the left and concave on the right side. i, 
28. Of the union of one curve line with another, _ 
: ‘The simplest combination of this kind is that of two equal 
4nd similar circular ares whose convexities are turned in opposite 
- directions, (Fig. 69.) These are readily united 69. 
 ither with or without the stereoscope, and form 
_ 41 the opposite direction. In either case, the a 
depth of the perspective is evidently dependant ears 
On the sum of the convexities of the two arcs. As will be shewn 
hereafter the form * the — 
