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entering it, even the naked eye along the sky. The effect pro- 

 duced is shown to be simply owing to this ; that the retina, under 

 such action, ceases to perceive in the spaces corresponding to its 

 blood-vessels and capillaries, so that they completely display them- 

 selves in the semblance of black bodies (or lines) ; and the law is 

 arrived at, that the images of external points which are painted on 

 the vessels and capillaries are not perceived when the retina loses 

 light from one point of space and receives light from another point of 

 space within a certain interval of time, or that the percipient points 

 lying in front of the vessels require a certain time to perceive. A 

 physiological hypothesis is suggested to account for this pheno- 

 menon, on the presumption that the " radial fibres," which project 

 from the layer of rods and cones and end in the limitary membrane, 

 are the ultimate percipients of light. 



It is pointed out how wonderfully close we may find the corre- 

 spondence between the microscopical and optical anatomy of the 

 retina. Each pair of identical fibres of the two optic nerves must 

 be regarded as one nerve. Another supposed anomaly to the sim- 

 plicity of nervous action being explained on anatomical principles, a 

 statement of ordinary optical nervous action is made, and a summary 

 evinces how the anomalies in visual experience are due to the com- 

 plex additions to a simple organ of sight. 



