308 lioijal Sucivhj. 



and it may roll with great rapidity. A rhythmical or recurrent 

 circulation of the retinal hlood is very frequent, and produces very 

 singular phenomena. We may produce the uncommon states of the 

 retinal circulation at pleasure, hy artifices described ; and it is shown 

 that it is the retinal circulation which is the cause of all the pheno- 

 mena which have been taken to prove spontaneous, vibratory, &c. 

 sensations of light. 



From these elementary facts being overlooked, fundamental errors 

 as to the conduct of the retina proper have prevailed on all hands. 

 When external light is so faint that the retinal light from blood- 

 pressure exceeds it, the middle of the retina is so occupied with retinal 

 light as to be, comparatively with other parts of the retina, unavail- 

 able for the usual purposes, and we do not see anything with direct 

 nearly so well as with oblique vision ; and this inefficiency of the 

 centre of the retina is not limited to the case of " stars of the last 

 degree of faintness" (Herschel and South), but all small objects 

 that are quite visible by "lateral" inspection appear to be " suddenly 

 blotted out" by the eyes being turned directly upon them. 



The rhythmical waves of light, or rhythmical progression of the 

 retinal blood (and the mode of movement of the retinal blood as 

 rendered by optical phenomena can be observed by other means), 

 may occur, in a certain sense, spontaneously, or may be produced at 

 will. The retinal circulation may be excited to show astonishing 

 luminous effects. 



Among other ways of causing a rhythmical or recurrent movement 

 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. M tiller and Brew- 

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

 cause of each of the phsenomena 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 pi'ove 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. 



