SENSE OF HEARING. 255 



347. 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 fol- 

 lowing experiment. 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 forwards, 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 phe- 

 nomenon 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 coloured paper lying upon a white 

 surface, the image of the coloured object will in a short time disappear, and 

 the white surface will be seen in its place ; the disappearance 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 neighbouring 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 surround- 

 ing 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 indistinct- 

 ness in a portion of the image. 



348. Under particular circumstances, we may receive a visual representa- 

 tion 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 con- 

 trary arise in the right and left divisions of the field, and immediately take 

 opposite directions. One trunk belongs to each eye, but their branches inter- 

 sect each other in the common field of vision. The explanation of this phe- 

 nomenon 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 mem- 

 brane which are not immediately covered by the vasa centralia are feebly 

 illuminated ; those parts, on the contrary, which are covered with those ves- 

 sels, 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."* 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. 



VI. Sense of Hearing. 



349. In the Ear, as in the Eye, the impressions made upon the sensory 

 nerve are not at once made by the body which originates the sensation; but 

 they are propagated to it, through a medium capable of transmitting them. 



* Mailer's Physiology, p. 1163. 



