VISION. 541 



tance having fallen on the blind-spot, or the point of entrance 

 of the optic nerve. 



It remains, then, to determine what part of the retina is 

 affected by light. The evidence that it is the layers of rods and 

 cones is convincing. It has been shown that parts of the retina 

 itself internal to these layers cast perceptible shadows, the con- 

 clusion that the rods and cones are the essential parts of the 

 sensory organ would be inevitable. 



The Laws of Retinal Stimulation.— It may be noticed that, 

 when a circular saw in a mill is rotated with extreme rapidity, 

 it seems to be at rest. 



If a stick on fire at one end be rapidly moved about, there 

 seems to be a continuous fiery circle. 



If a top painted in sections with various colors be spun, the 

 different colors can not be distinguished, but there is a color 

 resulting from the blending of the sensations from them all, 

 which will be white if the spectral colors be employed. 



When, on a dark night, a moving animal is illuminated by 

 a Hash of lightning, it seems to be at rest, though the attitude is 

 one we know to be appropriate for it during locomotion. 



It becomes necessary to explain these and similar phe- 

 nomena. Another observation or two will furnish the data for 

 the solution. 



If on awakening in the morning, when the eyes have been 

 well rested and the retina is therefore not so readily fatigued, 

 one looks at the window for a few seconds and then closes the 

 eyes, he may perceive that the picture still remains visible as 

 a. positive after-image; while, if a light be gazed upon at night 

 and the eyes suddenly closed, an after-image of the light may 

 be observed. 



It thus appears, then, that the impression or sensation out- 

 lasts the stimulus in these cases, and this is the explanation 

 into which all the above-mentioned facts fit. When the fiery 

 point pasang before the eyes in the case of the fire-brand stimu- 

 lates the same parts of the retina more frequently than is con- 

 sistent with the time required for the previous impression to 

 fade, there is, of necessity, a continuous sensation, which is in- 

 terpreted by the mind as referable to one object. In like man- 

 ner, in the case of a moving object seen by an electric flash, the 

 duration of the latter is so brief that the object illuminated can 

 not make any appreciable change of position while it lasts; a 

 second flash would show an alteration, another part of the 



