260 Mr.'WaUanon [Oct. 



and which has generally been referred to an insensibility in the 

 base of the optic nerve. 



I have frequently had occHsion to remark that objects are 

 perfectly seen oTily, and niost distinctly observed by the right 

 eye on the left side of the field of view, and by the left eye on 

 the right side of it/^ ' 1 tiike it fur granted,' in the firs* ijil ace, 

 that there is a cause existing toithin the eye why thiis should be 

 the case, and the thing to be ascertained is, where the real 

 seat of this cause is situated. Now, if the cause of our parti- 

 cular sensations were in the axis of the eye, every person would 

 appear to squint excessively when looking with both his eyes at 

 any object, placed but a trifling way beyond the point of distinct 

 vision, which is not the case; nor would the right eye then per- 

 fectly discover a point at the utmost limit towards the left hand, 

 because it could not be strained so far as that its axis should 

 form so great an angle with a perpendicular upon a right line 

 drawn through the centres of both the eyes. This much the 

 right eye can do howeVet towards the left hand, but not towards 

 the right, as Well Jis^triat' the left eye can do the same thing 

 few^iWtlife right hand, but not towards the left. If, therefore, 

 this is hot^^i*ftirhied by directing the axis of the eye towards 

 the point observed, it must be done by placing the eye in some 

 otber position uhich is capable of producing the effect. 

 ^ -'Leaving, this aroument as it stands, and referring to that 

 ^ixp^ritii'eftt Alluded to by which one of any three dark spots is 

 idst'od'dire'cting one eye towards that formed on either side of 

 the Centre, having the objects placed at an appropriate distance, 

 it will be found that the right hand object of the three is lost 

 by the right eye, and the one on the left hand by the left. From 

 this circumstance, it has been concluded thitt the base of the 

 cffltic 'nerve is insensible, because the undiscovered spot is 

 Siffectly'Opposite the place at which the nerve is inserted in the 



t'l'fiilt thfe teafp'ca'usfe is because the sensibility of the optic 

 ^^%ftlrea^y occdpied in conveying a particular sensation of 

 til^'ypdt^ttiw^rds which the eye is directed, and which falls 

 tlliflif the' opposite angle, with thaifexisKJf^he^'CJ^-tb^t formed 

 BVWi^'ii^ot which is lost. • MgJo,i«| vhbiff^! tnd// .r 



^ffcr%,^*hoWeVer, there is one gl"eat^ peculiarity to be' taken 

 lAS¥fefe df,'Whi^h; 'although remarkt^ble;, is an additional proof of 

 fh^^'trMh^bf;thede''ob^^Hati<inf^L -IM^ ^perfect pictm-e^t^ff ^the 

 to«SSi,'^feilJserVed by diredtin/y the ^yk tipbn the plano-convex 

 le^ wa^filtnid tabe situated exactly in the place where one of 

 IH^'^hffeWark spots mentioned above is lost. But this picture 

 (jpthfe moon is tefletted /m//i the eye upon the lens, while that 

 dtf^^'thfe dark spots 'Nkrhich is lost is opposed to the eye under 

 ^ liteie tingle With the' ^Jiis under which this picture i» 4rans- 



''^At^^J^jtW^'^*\^W^^^'^^ sen^i-decijMation pf^th^pptic nerve, in th^ Philo- 



