SENSE OF SIGHT. 451 



We must also inquire how it happens that, having two eyes, 

 we do not see double. In order to produce a single impres- 

 sion upon the central nervous organs of the brain, any object 

 whose image falls upon both eyes, and, consequently, forms 

 two impressions on the retina, must be depicted upon two 

 similar points in each retina. Seeing double, as in strabis- 

 mus, is caused by a want of symmetry between the part dis- 

 turbed in each retina (see p. 36). We must, however, add 

 that the necessity for the impression being made upon two 

 exactly similar points in the two retinae, is only the effect of 

 habit, and is by no means pre-established, or necessarily con- 

 nected with the anatomical arrangement of the eye, as im- 

 plied in J. Muller's nativistic theory. This theory has lately 

 yielded to the empirical theory, owing to Helmholtz's success- 

 ful experiments. Preparations made during observation with 

 a compound microscope, in which images are reversed, will 

 enable us to direct the movements of the eye without express 

 attention or care, though these are associated with a percep- 

 tion exactly opposed to our natural habit of vision. Persons 

 who are cross-eyed or who squint (afflicted with strabismus) 

 are not accustomed to blend in one the two images which 

 impinge upon non-coincident points in the two retinae, and 

 this habit becomes so strong, that immediately after the eye 

 has been restored to its natural position, there is diplopia or 

 double vision, though the image of any object be brought 

 to bear upon corresponding or coincident points on the retinae ; 

 the good effects of the operation for strabismus are slowly 

 developed. 1 



jective image (phosphainae, and those of the lower to the upper 

 part, etc.). This inversion also takes place when, instead of a 

 solid body (the extremity of the finger for the phosphainae) , the 

 reversed image is formed upon the choroid mirror (p. 447), which, 

 after reflexion, causes the rods to vibrate in the direction of their 

 axis. In this manner, the physical (optical) reversion, produced by 

 the intersection of the luminous rays at the nodal point, is formed 

 and cancelled. In short, the imaye, reversed by the optical conditions 

 of the eye, is restored by the physiological mechanism of the sensations 

 when carried to a distance from the point excited, in the same way as 

 the sensation of tingling of the skin, (see p. 53, Eccentricity of the 

 sensations), caused by a medullary congestion, extends far beyond 

 the part excited. A better illustration of this is seen in persons 

 who have lost a limb, and feel the sensation in the stump, spread, 

 as it were, to the extremity of the fingers." 



1 See E. Javal, Art. " Diplopie," in the " Nouv. Diet, de Med. 

 et de Ckirur. Prat." Vol. XI. p. 653. 



