240 PHILOSOPHICAL TRANSACTIONS. [aNNO 171 6. 



If, keeping both eyes open, you direct their axes towards o, a point nearer 

 than the usual place of the compound spectrum s, fig. 26, which point is in a 

 line from the nose n to the point s ; or in other words, if you look full at o, or 

 at the end of your finger held in o, the red and the blue (or purple spot) will 

 appear to be divided from each other after the manner represented at pr, in 

 fig. 27, where the red will appear to be on the right-hand, and the blue on 

 the left. 



To make plain what is meant by seeing the spectra p and r while we look full 

 at o, I beg leave to explain the distinction between looking and seeing ; that I 

 may the better show how this phenomenon proves that the sensation of differ- 

 ent colours is caused by rays differently refracted. 



1st Definition. — ^The optic axis is a line which, going through the centre of 

 the convexity of all the coats and humours of the eye, falls on the middle of 

 the retina, as a a or Aa, fig. IS. 



2nd Definition. — To look at any point, is to turn both eyes towards it in such 

 manner, that the optic axes making an angle at the said point as a, the rays 

 from a may have the optic axis for their axis, and, by their convergence upon 

 the retina after refraction in the eye, may paint the image of the said point 

 on the middle of the retina of each eye, where the optic axis in each eye 

 falls. 'uilji .••■ 



3rd Definition. — ^To see without looking, is to direct the optic axes to some 

 other place than to the point which is then seen ; and in such a case, the image 

 of the point seen will be projected on a part of the retina of each eye, where 

 the optic axis does not fall, namely either nearer to the nose n, as in fig. 26, at 

 the points of the retina marked nn; or farther from the nose than the middle of 

 the retina, as at 00 in fig. 'ig. 



Whatever is seen, by being looked at with both eyes, always appears single, 

 by reason of the communication between the middle of the retina in one eye, 

 and the middle of the retina of the other : there being no such communication 

 between any other part of the retina in one eye, and the correspondent part of 

 the retina in the other, when these correspondent parts are equally distant from 

 the nose. 



There is indeed a communication between the nervous fibres on the right-side 

 of the retina of one eye, and the nervous fibres on the right-side of the retina 

 of the other eye, and so of those on the left ; but no single object can be so 

 painted in each eye, as to have its image on the right or left part of one retina, 

 that communicates with the right or left part of the other, of the same size and 

 at the same time, as in the other ; because in whatever position the object is. 



