ON THE LIMITS OF SINGLE VISION, 12l7 



ever are not the only ones capable of receiving- images ; the commonly in 



space round the optic axes to a certain extent is also capa- ^^'^"^ *^^ 



ble of being afifected by them : these are the consequences 



deduclble from the facts already mentioned. But as the 



double image produces a single perception only when it falls 



on corresponding points, that is to say, which agree in their 



functions, whatever in other respects this part of the retina 



may be, the question is now to determine on what parts the 



images received will protluce single or double vision of the 



same object. As the muscles of the eyeballs sympathize 



together, so as constantly to arrange themselves in the same 



manner; awd this is generally so as for the eyes to receive 



the greatest number of parallel, or nearly parallel rays; the 



points of correspondence are usually at the summit of the 



optic axis, that is in parts which are in precisely the same 



situation on the retina, or which are symmetrical. In this Vision is single 



case vision is sino-le, as most phvsiolo"[ists have said, b^. y^''<='i''^'^ tvio 



_° ... . im^jressiijiis ar« 



cause the impressions are similar, being equal, and received receiv.d on si- 

 on similar parts. In rapid movements of objects however, nii^-'Taj'ts, 

 in those of the eyes produced by passions of the mind, in 

 the situation of objects within the distance of distinct vision, 

 and in certain positions of the head, how can we conceive, and sometimes 



that the impressions are made on pa;ts precisely svmmetri- ^^^^'^ ^" P^*"^* 

 1 s rr,, . • 1 n ■ 11. • 1- i' ""f^ l)ve<isel/ 



cal r This simple retlection had long incljned me to con- similar. 

 ceive, that images might be received on points of the 

 retina not syaimetrical without giving rise to double vi- 

 sion ; and experiment has fully convinced me of its truth. 



If, after having placed a lighted candle at the distance of ^^ double 

 eight or ten feet, it be looked at with one eye, and, the po- image pro- 

 ^ition of this eye being then tixed with a speculum, the j„„^,l^^,,^j'^^"g 

 other be moved inwardly or outwardly by pressing on it po^'t'o" of tii» 

 with one of the fingers, so as to incline the optic axes to one °^ ^'^ ^^'**' 

 another, we shall have a double iniage of the caudle. 



A double image is also obtained by inclining tiie two op- 

 tic , axes to one another at the same time, by turning the 

 eyes either inwardly or- outwardly, and inclining them to the 

 rays of the candle. The paralleham of the tnni.veibe axes 

 pf the eyeballs must be carefully pre-.erved, not <o confound 

 the effects occasioned by the inclination of the axes in ihe 



veilical. 



