SINGLE VISION. 273 



explanation, however, of this diplopia of the drunkard rests upon other 

 grounds. The effects of inebriating substances on the brain are to 

 interfere with all its functions; and most sensibly, with the voluntary 

 motions, which become irregularly executed. The voluntary muscles 

 of the eye partake of this vacillation, and do not move in harmony, 

 so that the impressions are not made on corresponding points of the 

 retina, and double vision necessarily results. 



Another hypothesis has been, that although a separate impression 

 is made upon each retina, in consequence of the union of the optic 

 nerves, the impressions are amalgamated, and arrive at the encephalon, 

 so as to cause but one perception. This was the opinion of Briggs, 1 

 and Ackermann; and at one time it was generally received. Dr. 

 Wollaston 2 supposed the consentaneous motion of the eyes to be con- 

 nected with the partial union of the optic nerves. The anatomical and 

 physiological facts relating to the union and decussation of these nerves 

 have already engaged us. By a reference to that subject it will be 

 found, that a true decussation takes place between them, yet that each 

 eye probably has its distinct nerve from origin to termination; and 

 that no such semi-decussation, as that contended for by Dr. Wollaston, 

 exists. These facts are unfavourable to the hypothesis of amalgamation 

 of impressions: besides, if we press slightly on the eye, we have a 

 double impression, although the relation of the optic nerves to each 

 other is the same ; and, moreover, the same explanation ought to apply 

 to audition, in which we have two distinct impressions, but only a single 

 perception: yet no one conceives that the auditory nerves decussate. 

 The fusion of the two images into one seems to be a mental operation. 



Another opinion has been maintained; that we do not actually 

 receive the perception of two impressions at the same time; and that 

 vision consists in a rapid alternation in the use of the eyes, according 

 as the attention is directed to one or other of them by accidental cir- 

 cumstances. Such was the opinion of M. Dutours. 3 A modification 

 of this view was entertained by M. Le Cat, 4 who asserts, that, although 

 the right eye is not always the most powerful, it is most frequently 

 employed; and Gall denies, that we use both eyes at the same time, 

 except in the passive exercise of the function. In active vision, he 

 asserts, we always employ one eye only, sometimes one and sometimes 

 the other; and thus, as we receive but one impression, we necessarily 

 see but one object. In support of this view, he remarks that in many 

 animals the eyes are situate at the sides of the head, so as not to be 

 capable of being directed together to the same object. In them, con- 

 sequently, one eye alone can be used; and he considers this a presump- 

 tion, that such is the case in man. He remarks farther, that in many 

 cases we use one eye by preference, in order that we may see better 

 as in shooting or in taking the direction of objects in a straight line ; 

 and that although, in other cases, both eyes may be open, we still 

 use but one. In proof of this, he says, if we place a small object be- 

 tween the eyes and a lighted body, and look at the latter, the shade 

 i 



1 Nova Visionis Theoria, Lond., 1685. 2 Philos. Transact, for 1824, p. 222. 



3 Memoir, presentees a 1' Academic cles Sciences, &c., t. iii. & iv. 4 Op. citat. 



VOL. I. 18 



