i; 



SENSE OF VISION ESTIMATION OF SIZE. 771 



axes. When the impression of a seal is looked at with this instrument, it 

 is converted into the representation of the seal itself; or, if the seal be looked 

 at, it presents the figure raised in relief, as in its ordinary impression. So, the 

 inside of a cup or basin appears as a solid convex body ; whilst the outside ap- 



ears depressed and concave. A bust regarded in front becomes a deep hol- 

 o\v mask ; whilst the interior of the cast of a face presents the appearance 

 of the face in its ordinary relief. A china vase, ornamented with colored 

 flowers in relief, seems like a vertical section of the interior of such a vase, 

 with hollow impressions of the flowers. The base of a brain seems concave, 

 like the interior of the base of the skull which is its reflex ; and the latter 

 seems convex and projecting, like the base of the brain. These and similar- 

 appearances are not always immediately perceived ; and some present them- 

 selves much more readily than others. Those converse forms which we are 

 accustomed actually to see, or which have a meaning that the mind can easily 

 apprehend, are those which are most readily perceived. Thus, the illusion 

 which may be produced with a bust or with the cast of a face, is not ob- 

 tainable even by a lengthened pseudoscopic contemplation of the real face, 

 which we cannot conceive of as thus "turned inside out." Another very in- 

 teresting fact is, that those to whom the illusion does not at first present 

 itself, usually find it suddenly come upon them after a little time, especially 

 if they should have directed their minds to the imaginary conception of the 

 object under its changed aspect. And, further, when the conversion has 

 taken place, the natural aspect of the object continues to intrude itself, 

 sometimes suddenly, sometimes gradually, and for a longer or shorter in- 

 terval, when the converse will again succeed it. This is due to the involun- 

 tary alternation of the attention, between the conception suggested to the 

 mind by the visual impressions derived from both eyes, and that which is 

 derived from either eye singly ; the latter, moreover, harmonizing with and 

 being strengthened by our recollection of the object as we have seen it before, 

 or (if it be new to us) by our notion of its natural appearance. A remarkable 

 illusion in respect to the direction of lines is exhibited in the accompanying 

 diagram (Fig. 275), constructed by Zolluer, 1 which if regarded vertically, and 

 still more if at an angle of 48 from the perpendicular with one eye, presents 

 the following peculiarities : Firstly, the dark longitudinal lines do not ap- 

 pear to be parallel, but to converge above and below alternately; secondly, 

 the two halves of each oblique stria, traversing a longitudinal line, do not 

 appear to be, as they really are, continuous with one another, but the lower 

 half seems to be displaced downwards; and thirdly, sometimes, though not 

 always, a stereoscopic effect is perceived, as though the lines were drawn on 

 a sheet of paper folded longitudinally. Hering 2 explains this by a train of 

 reasoning too long to be here inserted, but to the effect that all acute angles 

 under 60, on account of the natural curvature of the retina, appear less 

 than they really are. 



628. The persistence during a certain interval of impressions made upon the 

 retina, gives rise to a number of curious visual phenomena, which can be 

 here only briefly adverted to. The prolongation of the impression will be 

 governed, in part, by its previous duration : thus when we rapidly move an 

 ignited point through a circle, the impression itself is momentary, and re- 

 mains but for a short period : whilst, if we have been for some time looking 

 at a window, and then close our eyes, the impression of the dark bars trav- 

 ersing the illuminated space is preserved for several seconds. One of the 



1 Poggendorff's Annalen der Phys., Bd. ex, p. 500. 



2 Funke's Lehrbtich der Physiologie, vol. ii, 1866, p. 416. 



