... the field of view (G. Sehfeld) of each eye, which in the geometrical sense measures from right to left about 180°, appears much narrower. For the most left and right lying objects that one can still see and whose straight connection is a line through our eye, still appear to us as lying in front of us, as if their visual directions made an obtuse or rather right angle with each other. If one looks at the sky, such that no terrestrial objects of known position or size intrude in the visual field, then the bright field one has in front of oneself has about the diameter of a right angle from right to left, perhaps even less from top to bottom. It is as if you looked into the external world with your head at a certain depth (Helmholtz, 1892, p. 698, our translation).
... both the sun an my shadow as though they were not opposite but both were situated toward the front.
Design of the experiment
The histogram of the apparent fields of view λ is clearly bimodal. One (minor) mode is centered at the veridical value of 180° whereas the major mode is much broader and roughly centered at 90°. Values near λ = 0° (formally indicating true orthographic projection) are absent, though this is perhaps not surprising since the observers were required to draw something. There exist outliers up to 225° at the other side. The latter cases correspond to apparent field of views “extending beyond the ears”, evidently exceeding the veridical value and even the width of the physiological field of view;
The histogram of shape angles τ is very broad. Perhaps there is a (minor) mode at 180°, which is the case of the frontoparallel plane. However, the major mode is broadly distributed over the (roughly) 90°–150° range. Thus the shapes are typically much flatter than the veridical value of 90°.
The empirical findings
Apparently the normal population is far from being homogeneous, there exist both qualitative and quantitative variations that are surprisingly large;
The median apparent field of view of the 75 observers is only 56% of the veridical value, close to the top angle of the “cone of vision” according to the Greek authors and close to the value estimated by Helmholtz;
The median shape angle is 1.44 times the veridical value, thus the spherical shape is typically perceived as much flatter than it is, though certainly more curved than a frontoparallel plane. It is similar to the “vault of heaven” as experienced by normal observers when looking at a large extent of open blue sky (Minnaert, 1942);
Somewhat fewer than 10% of the observers had veridical apparent visual fields (defined as |λ−180°| ≤ 15°) and veridical apparent shape (defined as |τ−90°| ≤ 15°) experiences. Though only a minor group, the very existence of such observers is conceptually important.