516 THE HUMAN BODY. 



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

 scene were to be represented; by an 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 

 making fireworks: an ascending rocket produces the sen- 

 sation 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 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 ob- 

 jects, as distinguished from the power of recognizing dif- 

 ferences 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 stimula- 

 tion of one part of the retina from that of another part. 

 This power the visual apparatus possesses in a very higli 

 degree; while with the skin we cannot distinguish from one, 

 two points touching it less than 1 mm. (^ inch) apart, with 

 our eyes we can distinguish two points whose retinal images 



