452 SPECIAL PHYSIOLOGY. 



nomenon of irradiation. The vanishing of the dark image of the dot, 

 in the experiment above mentioned, was referred by Miiller, to a 

 power in the retina, of communicating to a smaller portion, a condi- 

 tion aifecting a larger part. Thus, when the retina is exposed to two 

 different impressions, one of which falls upon a larger, the other upon 

 a smaller portion of the membrane, the former impression is, after a 

 time, propagated to the whole of the surface, whilst the latter is no 

 longer perceptible. If, for example, one eye be directed, for a certain 

 time, upon a narrow slip of colored paper fixed upon a white ground 

 after a brief interval, the image of the former vanishes, the white 

 ground alone being visible ; this is most marked, when the lateral 

 portions of the retina receive the image (Purkinje and Brewster). 



In the exercise of the senses of touch, taste, and smell, we refer the 

 sensations excited, to the organ, or part, of the body where the stimulus 

 acts on the extremities of the nerves; but, as in hearing, so in sight, 

 the sensations are rapidly converted into perceptions, and are referred, 

 though far more definitely, altogether to the exterior, and, in the ex- 

 ercise of sight, actually to the external objects from which the rays of 

 light are given. The images formed on the retina, are never referred 

 by the mind, to the interior of the eye, where their existence is not 

 known to the untaught mind, and where, even when informed of the 

 fact, the mind is still unconscious of their presence. 



This outward projection of our visual sensations, is, by some, re- 

 garded as an ultimate fact incapable of explanation ; but others be- 

 lieve that it depends upon experience, gained by comparing the results 

 of vision, as regards our own bodies and external objects, with the 

 concomitant results afforded by the sense of touch, aided by movements 

 of the body. Vision, considered as a means of obtaining a knowledge 

 of the presence, form, color, position, and motion, of external objects, 

 is wholly dependent upon this outward projection of its impressions. 

 Even in excitement of the retina by pressure, electric shocks, or inter- 

 nal stimuli, the luminous impressions produced, are referred to the 

 exterior. 



The perception of objects in their erect position, through the agency 

 of an inverted image, is intimately connected with this outward projec- 

 tion of vision. The mind, in referring the luminous impressions in the 

 sensorium, to the objects whence the rays of light proceed, follows 

 these latter, as it were, from the retinal image, and views their several 

 focal points in the direction of certain imaginary lines, which are more 

 or less nearly perpendicular to the retinal surface. It has been shown 

 by Serre, in his experiments on the luminous spectra, produced by 

 pressure on the eyeball, and called by him phospMnes, that visual 

 impressions are projected from the retina, along certain lines, towards 

 a common centre in the eyeball, or optic centre, which he calls the 

 centre of direction, and which he locates in the middle of the crystalline 

 lens. Others, however, have variously supposed that these lines, which 

 they name lines of direction, meet in front of the lens, in the centre 

 of the pupil, or, behind it, in the centre of the eyeball. Having crossed 

 each other at the optic centre, these lines emerge from the refracting 



