FORMATION OF IMAGES IN THE EYE. 87 



bear upon it directly. Let us see, now, how far this fact is 

 capable of positive demonstration. 



If we examine the bottom of the eye with the ophthal- 

 moscope, we can see the yellow spot with the f ovea centralis, 

 apparently free from blood-vessels, and composed, as we know, 

 chiefly of those elements of the retina which are sensitive to 

 light. If, at the same time, we examine an image for which 

 the eye is perfectly adjusted, it will be seen that this image is 

 perfect only at the f ovea centralis ; and, if the object be re- 

 moved from the axis of vision, we see a confused image upon 

 the retina removed from the f ovea, at the same time that the 

 subject is conscious of indistinct vision. In the words of 

 Helmholtz, " it is only in the immediate vicinity of the ocular 

 axis that the retinal image possesses entire distinctness ; be- 

 yond this, the contours are less defined. It is in. part for this 

 reason that in general we see distinctly in the field of vision, 

 only the point that we fix. All the others are seen vaguely. 

 This lack of distinctness in indirect vision, in addition, de- 

 pends also upon diminished sensibility of tne retina : at a 

 slight distance from the fixed point, the distinctness of vision 

 has diminished much more than the objective distinctness of 

 retinal images." l 



At the point of penetration of the optic nerve, the retina 

 is insensible to luminous impressions ; at least, its sensibility 

 is so obtuse as to be entirely inadequate for the purposes of 

 vision. This point is called the punctum caecum; and its 

 want of sensibility was demonstrated many years ago by the 

 experiments of Mariotte, which we quote verbatim : 



" I fastened on an obscure "Wall about the hight of my 

 Eye, a small round paper, to serve me for a fixed point of 

 Vision; and I fastened such an other on the side thereof 

 towards my right hand, at the distance of about 2. foot ; but 

 somewhat lower than the first, to the end that it might strike 

 the Optick Nerve of my Right Eye, whilst I kept my Left 

 shut. Then I plac'd myself over against the First paper, and 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 88. 



