S. Rowley on Vision. 157 
Draw the straight lines C A, C’A, and perpendicular to them 
at A the two lines EG, IK. Through the points F, F’, taken 
in the lines CA, C’ A, ;; inch from C, C’, draw the lines FB, 
F’B, intersecting EG, IK, at 6, b’. Through the points F’ and 
d,—a point in IK where a line passing through 0b perpendicu- 
lar to CC’ intersects it—draw a line. In this line, where a line 
te B parallel to C C’ intersects it, take the point 
._ The distance of this point from B will be 4,°, inches. 
Now suppose the points OC, OC’, to represent the centers of 
the eyes ; points F', F’, the focal centers; A, B, D, the situa- 
tions of luminous points ; the lines CA, C’A, the optic axes 
produced ; the lines EG, IK, lines of intersection where two 
surfaces slightly convex toward the eyes, perpendicular re- 
spectively to the optic axes at their point of meeting, cut the 
ag of the axes; BF, BF’, lines of luminous rays passing 
om the point B through the points F,F’; DF’ the line 
‘Tay passing from D through F’ to the retina; 6 the 
point where the line BF intersects the line EG; and d, the 
point of intersection of DF’ with IK. Then, if the above 
enunciated proposition be true, to wit: that an impression 
made by a luminous point on the retina of the right or left 
eye becomes visible and is seen where a line drawn from it 
ugh a point'a little behind the center of the crystalline 
Uc axis of that eye at the place of its meeting with the axis 
of the other eye, the image of the point at B, will be seen 
left eye a little behind the place in which the image of the 
Point at D will be seen by the right eye. 
he experiment may be made thus. Take a thin rule, (as 
window at right angles to the rule, and parallel to the eyes. 
To obtain the former distance with accuracy itis necessary 
ahh two small objects, as two pins, placed equidistantly 
th cogs sides of the point of intersection of the bar by 
Shall €, distant from each other 4,°, inches, and when these 
nail be seen across (or lie in the axes exactly fixed upon) 
vision perpendicular of no variation is t perpendicular to the base of — 
ion, ae ; eee 
for the definition of which see note, p. 164. mee 
