THE EYE AS A SENSORY APPARATUS. 539 



A cat's eyes, for example, are said to "shine in the dark"; 

 and painters to produce moonlight effects always make the 

 bright parts of a picture relatively brighter, when compared 

 with things about them, than would be the case if a sunny 

 scene were to be represented ; by a relatively excessive use of 

 white pigment they produce the relatively great brightness of 

 those things which are seen at all in the general obscurity of 

 a moonlight landscape. 



The Duration of Luminous Sensations. This is greater 

 than that of the stimulus, a fact taken advantage of in mak- 

 ing fireworks : an ascending rocket produces the sensation of 

 a trail of light extending far behind the position of the bright 

 part of the rocket itself at the moment, because the sensation 

 aroused by it in a lower part of its course still persists. So, 

 shooting stars appear to have luminous tails behind them. 

 By rotating rapidly before the eye a disk with alternate white 

 and black sectors we get for each point of the retina on 

 which a part of its image falls, alternating stimulation (due 

 to the passage of white sector) and rest (when a black sector 

 is passing). If the rotation be rapid enough the sensation 

 aroused is that of a uniform gray, such as would be produced 

 if the white and black were mixed and spread evenly over 

 the disk. In each revolution the eye gets as much light as if 

 that were the case, and is unable to distinguish that this 

 light is made up of separate portions reaching it at intervals : 

 the stimulation due to each lasts until the next begins and so 

 all are fused together. If, while looking at the flame, one 

 turns out suddenly the gas in a room containing no other 

 light, the image of the flame persists a short time after the 

 flame itself is extinguished. 



The Localizing Power of the Retina. As already pointed 

 out a necessary condition of seeing definite objects, as distin- 

 guished from the power of recognizing differences of light 

 and darkness, is that all light entering the eye from one point 

 of an object shall be focused on one point of the retina. 

 This, however, would not be of any use had we not the faculty 

 of distinguishing the stimulation of one part of the retina 

 from that of another part. This power the visual apparatus 

 possesses in a very high degree; while with the skin we can- 

 not distinguish from one, two points touching it less than 1 

 mm. (- s inch) apart, with our eyes we can distinguish two 

 points whose retinal images are not more than .004 mm. 



