SENSE OF VISION STRUCTURE OF RETINA. 757 



that are most obviously thickened. On the other hand, there is no contin- 

 uous layer of nerve-fibres beneath the membrane limitans interna. The rods 

 are altogether deficient, their place being taken by the cones, which become 

 longer and narrower, and are closely packed. The course pursued by the 

 cone-fibres changes as compared with other parts of the retina from a radial 

 to a horizontal direction. At the f'ovea all the layers of the retina, with 

 the exception of the cones and external granules, diminish to a minimum. 

 It is not a little remarkable, that the point of the entrance of the Optic 

 nerve should be deficient in the power of receiving distinct visual impres- 

 sions ( 634) ; whilst the "yellow spot," the position of which according to 

 Landolt is 3.905 mm. to the outer side of the optic disk, and 0.785 mm. 

 higher, is the most sensitive portion of the entire Retina. 1 And hence it 

 seems unequivocally to follow, that these impressions cannot act primarily 

 upon the nerve-fibres, a conclusion which harmonizes with the fact, that 

 the fibres of the optic nerve are superimposed upon each other in the stratum 

 which they form, in such numbers that it is not conceivable that they should 

 be the primary recipients of luminous impressions, since their transparency 

 must allow rays of light to penetrate from one portion of the layer to 

 another. The bacillary layer was formerly regarded as a reflecting appa- 

 ratus, having for its purpose to stop the further passage of light, and to in- 

 tensify its influence on the true retina ; but since its connection with the 

 proper nervous elements of the retina has been established, there seems much 

 ground for believing (with Prof. Kolliker) that its rods and cones are the 

 primary recipients of luminous impressions, and that they communicate 

 their condition to the fibres of the optic nerve, by means of their own deli- 

 cate fibrous prolongations, which seem to come into more or less direct con- 

 nection with its ultimate ramifications. 



617. The limits of Human Vision, as regards the minuteness of the ob- 

 jects of which it can take cognizance, have been investigated by Professor 

 Ehrenberg, with the view of calculating the ultimate power of the Micro- 

 scope. 2 In opposition to the generally received opinion, Ehrenberg arrived 

 at the conclusion that, in regard to the extreme limits of vision, there is 

 little difference amongst persons of ordinarily good sight, whatever may be 

 the focal distance of their eyes. The smallest square magnitude usually 

 visible to the naked eye, either of white particles on a black ground, or of 



1 Aubert and Forster (v. Grafe's Arcliiv f. Ophth., Bd. vii, 1860, p. 152) have shown 

 that the acuteness of vision in receding from the fovea centralis does not diminish in 

 concentric circle?, but more quickly above and below, and more gradually towards 

 the out and the inside. They found also that the more remote any object was from 

 the optic axis the larger it must be to be recognized, and further, that a small object 

 in proximity to the eye can be recognized at a greater distance from the axis than 

 a larger object remote from the eye. The measure of acuteness of vision in general 

 use amongst ophthalmic surgeons is a series of letters, the thickness of which is one- 

 fifth of their height, and which are placed at such a distance that they subtend an 

 angle of 5'. The distance d at which such letters are still distinctly recognized di- 

 vided by the distance D at which they subtend an angle of 5' expresses the sharp- 

 ness of vision. Thus, if the letters C, or G, or B, be made of such a size that at '20 

 feet they subtend an angle of 5', and can be read at that distance, the vision of the 



experimenter is perfect, V = - or = 1 ; but if he can only see them at 10 feet, 



his vision = _ = _ ; if only at 5 feet his V = - = -, and so on. The absolutely 

 20 2 20 4 



smallest visual angle at which any object can under exceptionally favorable circum- 

 stances be seen subtends an angle of 50". (Hirschmann, 18(57.) 



2 Taylor's Scientific Memoirs, vol. i, p. 576. See also Helmholtz, Phy.<-iolog. 

 Opt ik.," 1860. 



