[253] Wundt seeks to explain all these illusions by the relatively stronger 'feeling of innervation' needed to move the eyeballs upwards,--a careful study of the muscles concerned is taken to prove this,--and a consequently greater estimate of the distance traversed. It suffices to remark, however, with Lipps, that were the innervation all, a column of S's placed on top of each other should look each larger than the one below it, and a weathercock on a steeple gigantic, neither of which is the case. Only the halves of _the same object_ look different in size, because the customary correction for foreshortening bears only on the relations of the parts of special _things_ spread out before us. Cf. Wundt, Physiol. Psych., 2te Aufl. ii. 96-8; Th. Lipps, Grundtatsachen, etc., p. 535. [254] Hering would partly solve in this way the mystery of Figs. 60, 61, and 67. No doubt the explanation partly applies; but the strange cessation of the illusion when we fix the gaze fails to be accounted for thereby. [255] Helmholtz has sought (Physiol. Optik, p. 715) to explain the divergence of the apparent vertical meridians of the two retinæ, by the manner in which an identical line drawn on the ground before us in the median plane will throw its images on the two eyes respectively. The matter is too technical for description here; the unlearned reader may be referred for it to J. Le Conte's Sight in the Internat. Scient. Series, p. 198 ff. But, for the benefit of those to whom _verbum sat_, I cannot help saying that it seems to me that the _exactness_ of the relation of the two meridians--whether divergent or not, for their divergence differs in individuals and often in one individual at diverse times--precludes its being due to the mere habitual falling-off of the image of one objective line on both. Le Conte, e.g., measures their position down to a sixth of a degree, others to tenths. This indicates an organic identity in the sensations of the two retinæ, which the experience of median perspective horizontals may roughly have agreed with, but hardly can have engendered. Wundt explains the divergence as usual, by the _Innervationsgefühl_ (_op. cit._. ii. 99 ff.). [256] Physiol. Optik, p. 547.