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The phenomena of this kind are always before us by daylight and 

 night. In every use of the organs of sight these effects may be 

 observed. In twilight, and into night, the pressure of the blood- 

 currents on the retina first equals and then excels the impression 

 made by the failing external light; and the whole circulatory 

 system may be seen, with proper attention, definitely figuring itself 

 in white or golden colour. A great concentration of light appears at 

 the middle of the retina, which requires a bountiful supply of blood, 

 and owing to the pointing of vessels towards the foramen centrale, 

 there is an apparent gyration of light currents round a darker pivot. 

 The whole conduct of the retinal circulation may be traced by the 

 blood-light. And the manner in which the blood flows through the 

 retina may be equable, or irregular and fitful ; it may be very slow, 

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

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

 singular phenomena. We may produce the uncommon states of the 

 retinal circulation at pleasure, by 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 



