380 VISION. LECT. XX. 



the cause of strabismus or squinting. An analogous phe- 

 nomenon is also produced with the sense of touch. To 

 effect this, cross the fore and middle finger of one hand, 

 and with their extremities touch a ball : you will experience 

 an illusion, and fancy that you touch two different balls in- 

 stead of one. 



Wollaston supposed that the unity of vision was attribu- 

 table to an anatomical cause. This philosopher thought 

 that the two optic nerves, at the point where they unite, in 

 passing out from the brain, and afterwards separate and 

 proceed to the eyes, divide in such a way that each nerve 

 forms half of both retina. From this semi-decussation of 

 the optic nerves, it happens that the right side of both 

 retina? is formed by the division of one nerve, and the left 

 side, by the division of the other nerve; so that all images 

 of objects out of the optic axis, are perceived by one and 

 the same nerve in both eyes, and the two exSited nerves, 

 consequently, furnish a single and complete image. By 

 this anatomical arrangement would be explained the phe- 

 nomenon which Wollaston and Arago observed in their 

 own persons, after long study, namely, that of seeing only 

 half of every object. We must, however, admit, that at 

 first, anatomical observations did not confirm this opinion ; 

 and that we may also oppose to it the fact of the single 

 sensation of sound by the two ears, by means of the two 

 acoustic nerves, which are perfectly distinct from each other 

 in their passage to the brain. 



Identical images painted upon the retina of two eyes by 

 a distant object, are such that there is no difference between 

 the perception produced by solid bodies in sculpture, or in 

 relief, and a drawing made on a flat surface, in which the 

 rules of perspective are followed. A picture representing 

 objects which we are accustomed to see at a certain dis- 



