308 T. W. Backhouse—Physiological Optics. 
way that the stick comes between one eye and one part of the 
pattern, and between the other eye and a corresponding part of 
_ the pattern is really of an indefinite character, with all its out- 
_ with a large optic angle, there was a slight appearance of curv- 
atu “ge straight lines on tha wall, conven to the point of 
_ sight; but, on the other hand, in two cases I su | the: 
were concave to the point of sight. This appearance of curva- 
ture in the straight lines was sometimes connected with an — 
appearance of curvature of surface, and sometimes with a flat 
surface. I cannot account for the illusion, which did not fol- 
low any perceptible law; it may perhaps have been caused — 
_ by the patterns used. os SS 
_ The largest optic angle I could observe with was 45° (dis- 
tance of phantom 3-4 inches). Stevens does not say how 
large an angle is necessary to produce the effect of a convex 
surface, so it may be that my want of any decided success 18 
owing to not having compassed a large enough angle. ee 
n these experiments the nearest point at which I could see 
an object in perfect focus was 45 Shieh (optic angle 31°°0), and 
_ if my theory is correct it may be that the suspicion of convex” 
_ ity with large optic angles is owing to the stick not being 2 
oe focus ; i. e., to its being impossible for me to obtain a focal ad- 
_ justment corresponding to the axial adjustment, though itis 
_ very probable that the strain on the ciliary muscles in endeav- 
coring to bring it- into focus is sufficient to prevent the judg- 
ment of distance by means of the optic angle from being mate 
rarpe that through the ciliary muscles. 
ge a r the phantom wall and the s 
about over 
