450 ORGANS OF THE SENSES. 



consists in the fact that we always see objects upright, and in 

 their natural position, although their image appears upside- 

 down on the retina; this may be, however, readily explained. 

 We see objects erect and not upside-down because our mind 

 carries outwards every impression made upon the retina, and 

 conveys it in the same direction that the rays of light must 

 necessarily follow, according to the laws of optics, in order to 

 produce an impression on any part of the sensitive mem- 

 brane ; in other words, every part of the field of the retina 

 has a part of the external visual field corresponding to it, and 

 these two are so closely connected, that what takes place in 

 one produces the corresponding effect in the other. Thus, 

 when we look at an object so long that the retina becomes 

 fatigued, and the image remains upon it, even when the eyes 

 are closed, the image still appears upright, and not upside 

 down. It is impossible to decide whether this is only the 

 effect of habit and of the education, of the senses, for cases 

 have been known in which persons blind from their birth 

 have seen objects upright and not upside-down, from the 

 very moment that they were able to see. 1 



1 We have already protested (see p. 447) against the ancient 

 theory that the retina simply resembles a screen. We have seen 

 that it is not enough to trace the passage of a ray of light until it 

 reaches the retina, but that we must examine this after it has 

 entered the sensitive membrane. This examination, made as 

 above (p. 447), gives us exactly what we need for the purpose of 

 explaining why we see objects upright, although their image on the 

 retina is upside-down. We know that mechanical pressure of one 

 part of the retina gives rise to a luminous image (phosphaina, 

 p. 443), which appears to be situated on the side of the field of 

 vision opposite to that on which the compression is made (see 

 Serre d'Uzes, " Essai sur les Phosphenes ou Anneaux lumineux 

 de la Retine." Paris, 1853.) " The position of the subjective 

 image of the phosphainae," says Rouget, " which image is dia- 

 metrically opposed to the region of the retina excited (although 

 this image is entirely independent of the optical phenomena of 

 vision), proves that the impressions communicated to the extremi- 

 ties of the nerves of the retina by the intermediation of the rods 

 (see p. 447) are carried beyond the eye in the direction of the prolonged 

 axes of the rods. The prolonged axes cross each other at the 

 centre of the curve of the retina (in the eye), the rods being 

 arranged according to the rays of this curve. After their intersec- 

 tion, they are outside of the eye, in the part in wliicl the subjec- 

 tive image is produced, the direction being the reverse of that of 

 the rods themselves, the prolonged axes of the rods of the upper 

 region of the retina corresponding to the lower part of the sub- 



