[257] "We can with a short ruler draw a line as long as we please on a plane surface by first drawing one as long as the ruler permits, and then sliding the ruler somewhat along the drawn line and drawing again, etc. If the ruler is exactly straight, we get in this way a straight line. If it is somewhat curved we get a circle. Now, instead of the sliding ruler we use in the field of sight the central spot of distinctest vision impressed with a linear sensation of sight, which at times may be intensified till it becomes an after-image. We follow, in looking, the direction of this line, and in so doing we slide the line along itself and get a prolongation of its length. On a plane surface we can carry on this procedure on any sort of a straight or curved ruler, but in the field of vision there is for each direction and movement of the eye only one sort of line which it is possible for us to slide along in its own direction continually." These are what Helmholtz calls the 'circles of direction' of the visual field--lines which he has studied with his usual care. Cf. Physiol. Optik, p. 548 ff. [258] Cf. Hering in Hermann's Handb. der Physiol., iii. 1, pp. 558-4. [259] This shrinkage and expansion of the absolute space-value of the total optical sensation remains to my mind the most obscure part of the whole subject. It is a real optical sensation, seeming introspectively to have nothing to do with locomotor or other suggestions. It is easy to say that 'the Intellect produces it,' but what does that mean? The investigator who will throw light on this one point will probably clear up other difficulties as well. [260] Examination of Hamilton, 3d ed. p. 283. [261] Senses and Intellect, 3d ed. p. 183. [262] Exam. of Hamilton, 3d ed. p. 283. [263] Senses and Intellect, p. 372. [264] Vierfache Wurzel des Satzes vom zureichenden Grunde, pp. 52-7. [265] Psychol. als Wissenschaft, § 111. [266] Psychol. als Wissenschaft, § 113. [267] Lehrbuch d. Psychol., 2te Auflage, Bd. ii. p. 66. Volkmann's fifth chapter contains a really precious collection of historical notices concerning space-perception theories. [268] Why talk of 'genetic theories'? when we have in the next breath to write as Wundt does: "If then we must regard the intuition of space as a product that simply emerges from the conditions of our mental and physical organization, nothing need stand in the way of our designating it as one of the _a priori_ functions with which consciousness is endowed." (Logik, ii. 460.) [269] P. 430. [270] Pp. 430, 449. [271] P. 428. [272] P. 442. [273] Pp. 442, 818. [274] P. 798. Cf. also Popular Scientific Lectures, pp. 301-3. [275] P. 456; see also 428, 441. [276] P. 797. [277] P. 812. [278] Bottom of page 797.