FORMATION OF THE IMAGE UPON THE RETINA 849 



that the image becomes the smaller, the less this angle. Obviously, 

 the latter must vary directly with the size of the object and inversely 

 as its distance. Thus, if we gaze first at the moon and then at a more 

 distant but much larger fixed star, the visual angle formed by the rays 

 from the moon is much larger, because its relative proximity to the 

 eye more than makes up for its smaller size. 



This inversion of the image may be conveniently demonstrated 

 by observing a landscape upon the ground glass of a photographic 

 camera. Quite similarly, we may employ the eye of an albino rabbit 

 which contains no choroidal pigment and in which, therefore, the 

 image may be seen through the transparent sclerotic coat. The 

 question may then be asked, why do we not perceive objects upside 

 down? Our correct interpretation of spatial relationships is gained in 

 the course of time by experience and in consequence of the association 

 of various sensory impressions. In other words, our psychic mech- 

 anism is adjusted in such a Way that it conforms absolutely to this in- 

 version of the image. Consequently, any ray of light striking the 

 retina below, is invariably regarded as having arisen from a luminous 

 point situated in the upper visual field. Quite similarly, any stimula- 

 tion of the upper expanse of the retina is correctly interpreted as 

 having originated in the lower visual field, and so on. 



The fixed character of our spatial associations may be proved in 

 different ways. Thus, we have previously observed that the mechan- 

 ical stimulation of the retina gives rise to luminous sensations or phos- 

 phenes, which are invariably referred to the visual field opposite the 

 seat of the stimulation. The reason for this is that these elements are 

 invariably stimulated by rays which are projected along these particu- 

 lar secondary lines. In localizing these retinal stimuli in space, it 

 may be imagined that we are guided by the local signs previously 

 established by them in the visual center. Like the receptors of the 

 skin, each retinal element may be assumed to be connected with a 

 particular central neurone which in the course of time has become 

 adapted to a perfectly definite sensation. Our psychic interpretation, 

 therefore, corresponds, as it were, to a reversal of the rays of light, i.e., 

 the stimulated points upon the retina may be imagined to emit rays 

 which pass in a straight line through the nodal point and form an 

 imaginary image in space in accordance with their secondary axes. 



Another good illustration of this general fact is obtained whenever 

 objects are held so close to the eye that the ordinary inverted image 

 must give way to an erect shadow (Fig. 450). To accomplish this 

 end, a card with a pin-hole is held at a distance of about 3 cm. in front 

 of the eye, i.e., within the near point of vision. If a pin is now moved 

 slowly upward in front of the pupil and as close as possible to the cor- 

 nea, the pin appears to enter the visual field from above. The same 

 result is obtained if the object is moved along any other meridian of 

 the cornea. Since the pin-hole lies inside the near point of this eye, 

 it is converted into a source of light which widely illuminates the ret- 



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