vni OCULAR MOVEMENTS 419 



triangle appears greater than the two angles of the 'base ; when 

 we look at a square, the vertical s,ides seein longer than the hori- 

 zontal sides. Helmholtz observed that in making a free-hand 

 drawing, by simple measurement with the eye, of a square on a 

 .surfa.ee perpendicular to the line of sight, it is a common error to 

 draw the vertical lines shorter than the horizontal Lines. This 

 error over-estimation of the vertical sides varies between ,.',, 

 and -.^ of the total length. 



These and other more complicated observations, omitted here 

 for want of space, show (according to Hering) that the power of 

 discriminating size by visual measurement does not depend on the 

 muscular and innervating sensations necessarily associated with 

 the ocular movements, but is essentially dependent on the " local 

 signs " in the retina. 



By the local signs or spatial appreciation of the retina is 

 meant its power of recognising and distinguishing the position 

 of the various points of objects in the visual field. To explain 

 this power we must assume that the excitation of each retinal 

 cone is insulated and conducted separately to the cerebral cortex, 

 and is there separately perceived and differentiated from impulses 

 of the same quality and intensity from other cones. In the same 

 way, we are able to distinguish and recognise the site of contacts 

 in the cutaneous surfaces where tactile sensibility is most delicate. 

 The hypothesis of local signs invoked to explain the ability to 

 discriminate between elementary sensations of contact (Chap. I. 

 p. 43), is also applicable to the discrimination of elemeutary visual 

 sensations produced by identical stimulation of the various 

 elements of the retina, and localised in different parts of the 

 visual field. Just as local signs are differently developed in 

 different regions of the skin, so local retinal signs are most 

 developed at the fovea the region of distinct and direct vision 

 and decline gradually thence towards the ora serrata, where vision 

 is blurred and indirect. Practically, therefore, the development of 

 spatial appreciation in the retina is parallel with visual acuity, 

 since both decline from the centre to the periphery, but theoretic- 

 ally they are quite distinct : spatial appreciation determines local- 

 isation, visual acuity, the quality and intensity of visual 

 sensations. 



Guillery (1899) attempted to differentiate the sense of form 

 from the visual acuity and the spatial appreciation of the retina 

 the faculty, that is, of recognising and correctly judging the 

 form of objects. This power depends only partially upon the 

 processes by which we estimate size, and is a psychical estimation 

 fundamentally connected with past experience and practice. It 

 follows from Guillery 's investigations that the power of recognising 

 more or less simple or complete forms does not depend on their 

 linear extension; in other words, our judgments of the form of 



