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of this blood, is that by simple fatigue of the retina by overstraining 

 the sight, when the retina, more or less suddenly (or after a few 

 oscillations), becomes flooded with blood, and complete obliteration 

 of all objects having less than a certain luminosity ensues. This 

 circumstance has misled Brewster and Purkinje, separately, into the 

 belief that they had discovered that a sensation excited in one por- 

 tion of a retina may be " extended" or "irradiated" to an adjacent 

 portion. Other cases which are imagined by J. Miiller and Brew- 

 ster to support this view are subjected to examination; the real 

 cause of each of the phenomena mentioned being pointed out. 

 Some peculiar effects of retinal light are given ; and it is determined 

 that the rigid correspondence of the limits of sensation with those of 

 the painted image, is a physiological law literally absolute. 



Unsuspected difficulties of a solitary eye, and certain well-known 

 phenomena are explained upon the foregoing principles. 



May sensation be excited in the trunk of the optic nerve, or centri- 

 fugally ? 



The arguments which have been presumed to prove the affirma- 

 tive are shown, one by one, to be fallacious, while there is pre- 

 sumption of a negative sort. Observations are offered as to the cor- 

 rect explanation of various physiological points which have been 

 otherwise interpreted, and reputed physiological contrasts of colour 

 are considered. >. 



Images of external objects are painted on the limitary membrane, 

 and perceived by the radial fibres. 



This head commences with the quotation of a passage from Sir 

 David Brewster's 'Optics' which he offered towards an explanation 

 of the difficulty of seeing a very faint star by direct vision ; and it is 

 shown that the retina is not liable, as Brewster imagines, to be 

 thrown into a state of " undulatory" perception by our looking 

 through the teeth of a "fine comb" or through a single " narrow 

 aperture." The paper points out that the effect observed in these 

 circumstances is produced by our looking near the edge of any body 

 whatever, provided, and only then, that the object move, be it never 

 so little, across the eye's axis. It shows that the same effect is pro- 

 duced by light radiating from a point, by a flame, by lenses, curved 

 reflectors, whilst they are in the act of moving across the eye's axis ; 

 or by the movement of the eye itself, merely in relation to the light 



