192 ox THE SENSES. 



objects. This mode of interpretation once established, we are thenceforth its 

 slaves ; we can never again free ourselves from its influence nor conceive of a 

 sensation of light as purely subjective, and without the idea of a causative 

 external object to which it is referable. 



Yet this reference of the sensations of light to the outward world is still not 

 a complete act of vision ; for this there is further required the perception of the 

 local relations of the external things to which we refer the sensations. And 

 here the organ of sight offers a complete analogy with the organ of touch. We 

 must conceive of our retina as a mosaic of sensitive points, just as our outward 

 skin was shown to be ; according to the specific coloring which each of these 

 sensitive points imparts to the sensation called forth by it, does the mind recog- 

 nize the place in outer space at which it is to seek the object of the sensation. 



In conception the space embraced by the field of view is distributed into just 

 so many compartments as the mosaic of the retina comprises points of sensation ; 

 if one of the latter be struck by an impression of light, we refer the sensation 

 to that compartment of the field of vision which corresponds to the point struck. 

 If a series of points of the retina which lie in a right line be struck simulta- 

 neously we refer the impressions to those compartments of the ideal mosaic which 

 lie in the corresponding right line. If an image of a sphere, of the moon, for in- 

 stance, be delineated on the retina, the points of the retina which are struck lie 

 in a spherically bounded surface contiguous to one another, and the same is 

 the case in the field to which our idea transfers the sensations belonging to the 

 several points ; hence it is that in the first case we see a right line ; in the second, 

 a sphere. If the retina be simultaneously struck by light at two points separated 

 from one another, each of these impressions connects itself with the appropriate 

 idea of locality. We see two distinct points of light and comprehend their 

 severance from one another, since we are conscious of the mosaic particles of the 

 retina remaining unaffected which are situated between the two that are struck ; 

 we mentally enumerate these, and from their number form a judgment of the 

 intervening distance. If we observe two objects, one after the other, the image 

 of the first of Avhich covers four, that of the second eight sensitive points on 

 the retina, we estimate the magnitude of each of them and their relative pro- 

 portion according to the number of sensitive points which are struck, or rather 

 according to the number of the compartments embraced in our ideal field of 

 vision, which in both cases we have filled up with our sensations. In this 

 manner we arrive at the perception of the form and size of observed objects, 

 but always without being conscious of the meditator between the object on the 

 one hand and the sensation and idea on the other, the image, namely, on the 

 retina with its local relations ; always without attention to the mental operation 

 by which we have once laboriously learned to clothe the simple sensations with 

 the ideas of form and magnitude. 



, For a clearer explanation of the local perception, we have called to our aid the 

 hypothesis that the retina represents a mosaic of sensitive points ; this hypothesis 

 it is proper that we shoidd now a liitle more closely examine. Physiology must 

 take it for granted that each of the countless minute filaments of the optic nerve, 

 which, packed together in its stem, are afterwards dispersed over the retina, there 

 terminate in a peculiar apparatus ; that the terminal apparatus of all these fila- 

 ments lie, arranged near one another, in the plane of the retina, like the pieces of 

 a delicate mosaic work ; and that each apparatus represents in reality a sensitive 

 point endued with the nature and properties which we have supposed in our ex- 

 planation of the local perceptions of the organ. If this be the case, everything 

 is easily explained, and the results of the most recent microscopic investigations 

 permit us to conclude with almost entire certainty that the retina is, in fact, such 

 , an anatomical mosaic of distinct nerve-ends, that the rod- shaped and conical 

 appendages of the nerves in the hindermost layer of the retina form respectively 

 just such sensitive points as we have supposed. Let it be assumed, then, that 



