THE FUNCTION OF THE RETINA IN VISION. 851 



The proof that it is really the optic-nerve entrance that is not sensitive is 

 furnished by the following observations: (i) Donders, by means of a mirror, 

 threw a small image of a flame directly on the optic-nerve entrance of another 

 person, who had no sensation of light. This appeared, however, as soon as the 

 image was displaced to the neighboring portions of the retina. (2) If Mariotte's 

 experiment be combined with the experiments that yield entoptic phenomena at 

 the optic-nerve entrance, the latter coincide with the blind spot. 



In order to determine the form and apparent size of the blind spot in one's 

 own eye, the head should be placed at a distance of about 25 cm. from a piece 

 of white paper. Then a small point should be fixed with the eye, and the position 

 of the blind spot on the paper determined by moving a white feather about in 

 various directions, making a mark wherever its point first becomes visible. In 

 this way the blind spot may be mapped out, and it 

 will be found to have an irregularly elliptical form, 



from which processes extend, representing the insensi- o V~\ (^ 



tive origins of the large central vessels of the retina. CL t<J l^s 



Mariotte concluded, from his experiment, that the 

 choroid, which is perforated by the optic nerve, is the 

 light-perceiving membrane, as the nerve-fibers are p] / r\\ 



nowhere absent from the retina. ^ \^ / 



The blind spot in the eye causes no appreciable 

 defect in the visual field. As the area is not excited 



by light, a black spot cannot appear in the visual field, Q. L-* j 



for the sensation of black presupposes the presence 5 1 1 



of retinal elements, which, however, are absent at the 

 blind spot. The circumstance that, despite the in- 

 sensitive spot, no unoccupied spot in the visual field is perceived is due to psy- 

 chical action. The unoccupied part of the field, corresponding to the blind 

 spot, is probably filled out by a psychical process. Hence, when a white point 

 on a black surface disappears, the entire surface appears black. A white surface 

 of which a black point falls upon the blind spot appears entirely white, a printed 

 page grayish throughout, etc. In the same way, parts of a circle, the middle parts 

 of a long line, the middle portion of a cross are probably supplied. Such images, 

 though, that cannot be reconstructed on a basis of probability are not completed, 

 for example the end of a line, or a human countenance. Under other conditions 

 a phenomenon contributes to the filling out of the empty space that has been 

 designated the "contraction of the visual field." This becomes clear if the letter 

 e is made to disappear from among the nine adjoining letters; the three letters of 

 each side are no longer seen in a straight line, but b, f, h, d are drawn in toward 

 e. The surrounding portions of the field seem to stretch out over the position 

 of the blind spot and help to replace it. 



The outer segments of the rods and cones possess rounded con- 

 tours ; they are placed close together, but there must be spaces between 

 them (corresponding to the interspaces between circles placed in con- 

 tact). These spaces are insensitive to light, so that the retinal image 

 is constructed like a mosaic of small round stones. The diameter of a 

 cone in the yellow spot measures from 2 to 2.5 //. If two closely sit- 

 uated points form images on the retina, they will be perceived as isolated 

 points, provided that the images fall upon two different cones. A dis- 

 tance of from 3 or 4 to 5 .4 // between the images on the retina is sufficient 

 to enable them to be seen separately, as they will then fall on two neigh- 

 boring cones. If the distance is so diminished that both images fall 

 on one cone, or one on a cone, and the other on an interspace, only one 

 point will be perceived. In the peripheral portions of the retina, the 

 images must be still further apart in order to be perceived separately. 



As the rounded ends of the cones are not placed in exactly straight lines, 

 but so that a row of circles is adapted to the interstices of the succeeding row, 

 exceedingly fine dark lines, drawn parallel, appear to have alternating twists, 

 as their images must fall on the cones alternately to right and to left. In the same 

 way every straight edge of an object appears wavy when its retinal image is moved 

 across the retina with moderate rapidity. 



