SENSE OF VISION. 



amidst the leaves on a tree, except by the difference of their form. Several 

 distinct varieties of this affection may be distinguished, however; these have 

 been classified by Seebeck and Wartmann. 1 



895. Amongst other curious phenomena of Vision, is the vanishing of images 

 which fall at the entrance of the optic nerve; as is shown in the following ex- 

 periment. Let two black spots be made upon a piece of paper, about four or 

 five inches apart ; then let the left eye be closed, and the right eye be strongly 

 fixed upon the left hand spot. If the paper be then moved backwards and for- 

 wards, so as to change its distance from the eye, a point will be found at which 

 the right hand spot is no longer visible; though it is clearly seen when the paper 

 is brought nearer or removed further. In this position of the eye and object, 

 the rays from the right hand spot cross to the nasal side of the globe, and fall 

 upon the point of the retina which has just been mentioned. The phenomenon 

 is not confined to that spot, however; nor is it correct to say, as is sometimes 

 done, that the retina is not sensible to light at that point; since, if such were 

 the case, we should see a dark spot in our field of view whenever we use only 

 one eye. The fact is, that a similar phenomenon may occur under somewhat 

 different conditions, in any division of the retina, especially in its lateral parts. 

 Thus, if we fix the eye for some time, until it is fatigued, upon a strip of colored 

 paper lying upon a white surface, the image of the colored object will in a short 

 time disappear, and the white surface will be seen in its place; the disappear- 

 ance of the image, however, is only of a few seconds' duration. The truth seems 

 to be, that there is a tendency in the retina to the propagation, over neighbor- 

 ing parts, of impressions which occupy a large proportion of its surface; and 

 that this tendency is the strongest around the point at which the optic nerve 

 enters, so that the state of this part will generally become similar to that of the 

 surrounding portion of the retina. Hence, when we are using one eye only, we 

 do not perceive any dark spot in the field, but only a certain degree of indis- 

 tinctness in a portion of the image. 



896. Under particular circumstances, we may receive a visual representation 

 of the retina itself, as is shown by the experiment of Purkinje. " If in a room 

 otherwise dark, a lighted candle be moved to and fro, or in a circle, at a distance 

 of six inches before the eyes, we perceive, after a short time, a dark arborescent 

 figure ramifying over the whole field of vision; this appearance is produced by 

 the vasa centralia distributed over the retina, or by the parts of the retina 

 covered by those vessels. There are, properly speaking, two arborescent figures, 

 the trunks of which are not coincident, but on the contrary arise in the right 

 and left divisions of the field, and immediately take opposite directions. One 

 trunk belongs to each eye, but their branches intersect each other in the com- 

 mon field of vision. The explanation of this phenomenon is as follows : By 

 the movement of the candle to and fro, the light is made to act on the whole 

 extent of the retina, and all the parts of the membrane which are not immedi- 

 ately covered by the vasa centralia are feebly illuminated ; those parts, on the 

 contrary, which are covered with those vessels cannot be acted on by the light, 

 and are perceived, therefore, as dark arborescent figures. These figures appear 

 to lie before the eye, and to be suspended in the field of vision/' 2 We have 

 thus another demonstration of the fact that, in ordinary vision, the immediate 

 object of our sensation is a certain condition of the retina, which is excited by 

 the formation of a luminous image. 



' Muller's "Elements of Physiology" (Baly's Translation), p. 1213; and Taylor's 

 "Scientific Memoirs," vol. iv. p. 156, et seq. 

 2 Muller's "Elements of Physiology" (Baly's Translation), p. 1163. 



