482 Royal Society. 



At the distance of about 

 S\ inches I see the two 

 oblique lines equally well, 

 but not so distinctly as at 

 the distance of 10 inches. 



At the distance of 12 

 inches I see the two oblique 

 lines with much about the 

 same distinctness as that 

 with which I see them at 

 the distance of 8£ inches. 



It thus appears that I 

 cannot see either of the 

 oblique lines with perfect 

 distinctness and definition 

 at any distance ; but that 

 I can see them both simultaneously distinctly enough at any di- 

 stance from 8^ inches to 12. At a nearer distance than 8| inches, 

 or a further distance than 1 2 inches, the distinctness diminishes, and 

 that equally for the two lines. 



I cannot by any adjustment of my eyes vary the distinctness with 

 which I see the oblique lines at a given distance. 



The preceding analysis of my sight shows that my eyes are not 

 monostiymatic, that is, are not capable of collecting all the rays of 

 light which enter them to one exact focus. It shows, on the contrary, 

 that my eyes are distiymatic, that is, they have each two distinct 

 foci to which they bring the rays, viz. one focus for horizontal rays, 

 and one for vertical rays. 



The preceding analysis also shows that my eyes do not ppssess any 

 intrinsic power of adjustment whereby they can bring to foci rays 

 diverging from a nearer or further distance than the two distances 

 above specified for horizontal and for vertical rays. 



It is true that I can see the different objects in a room distinctly 

 enough without the aid of glasses, and that in the street or open 

 country I can see objects distinctly enough for all practical purposes 

 with the aid of concave glasses Nos. 2 and 3, but, critically speaking, 

 the definition is far from being exact. 



Directing my eye to an object 2 or 3 feet from me, I see it 

 distinctly enough whilst an object in the same field of view at the 

 distance of 1 or 1 2 feet is at the same moment seen very indistinctly. 

 If now, I direct my eye to the object at the distance of 10 or 12 feet, 

 I see it distinctly enough, but the object at the distance of 2 or 

 3 feet now appears very indistinct. 



This is commonly considered an evidence of adjustment of the eye 

 to the two different distances. There is, however, no real intrinsic 

 adjustment in the case. I see distinctly enough, either the nearer 

 or the more distant object, merely because by directing my eye to 

 it, its image falls on the central and most sensitive part of the retina, 

 whilst the image of the other object falls on the circumferential and 

 least sensitive part of the retina. 



It is to be observed that at neither the nearer nor the further 



