128 "N THE LIMITS OF SINGLE VIStOW* 



verllcal plane vvitli those owing to their inclination in the 

 horizontal plane. 



If, one of the two eyes btins^ shut, the other be clirected 



to some object placed above or below the candle, and, after 



it is hxed in this situation, the other eye be suddenly 



opened, we shall have a double ima^e of the candle. 



Theaxesmust A slii>ht inclination of the optic axes in the vertical plane 



fori:: a grrater ojves rise to the double Iniaije; but it will not occur in the 



an^lo. horizon- " , , i " ■ - . t , • ,■ ■ n 



t:.Hy ihaa ver- h'.nzontal plane, unless there is a considerable inclination of 



licaily. the axes. 



Tjrning the The double image may be obtained in the horizonial 



VccIlI ISO IS to plane, without employing any violence on the eye, that caa 



iect very ')b- ^^'^^ "^ *<^ suspect a change in the figure of this organ. It 



liquely suffi- is sufficient for this to look very obliquely at a conspicuous 



object placed level with the eyes; which may be done by 



turning the bead wdiile looking at it, till the muscles are 



unable to give the eyeballs a parallel direction, so that they 



are obliged to receive rays that reach points of the retina 



very differently situate. We may even obtain the double 



image in the oblique plane, by a certain inclination of the 



head : but in the vertical plane it can be obtained only by 



mechanical means. 



The corres- These facts, confirmed by a great number of experiments 



por.diag points repeated and varied in several ways, which it would take up 



admit of some ^ ^- i i i , 



tariaiion. ^^9 much room to mention here, not only prove, that nature 



has allowed a certain latitude in the law she has imposed on 

 herself with respect to corresponding points ; and that it i» 

 not absolutely neceffary, that the points of the retina on 

 which impressions are made should always be similarly situ- 

 ate; but also that there are points on the retina, which, 

 taken at different distances from the optic axis, may pro- 

 duce double images or not, according to the direction in 

 which they arrive at the eye. Thus in the transverse plane 

 the optic axes must be greatly inclined, to give rise to the 

 double image ; while a sli-ht inclination produces this plie- 

 The field of nomenon iu the vertical plane. These general hmits of the 



single vision an ^ jj ^^ single vision, or of the area of corresponding points, 

 ellipsis* . ' . . , 



give the elements of an ellipsis, the longer axis of which is 



parallel with the transverse axis of the eye, and the shorter 



parallel 



