274 THE HUMAN BODY 



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 extin- 

 guished. 



The Localizing Power of the Retina. As already pointed out 

 a necessary condition of seeing definite objects, as distinguished 

 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 

 focussed 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 



i 



FIG. 95. 



power the visual apparatus possesses in a very high 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 distin- 

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

 (.00016 inch) apart. The distance between the retinal images of 

 two points is determined by the " visual angle " under which 

 they are seen; this angle is that included between lines drawn 

 from them to the nodal point of the eye. If a and 6 (Fig. 95) are 

 luminous points, the image of a will be formed at a' on the pro- 

 longation of the line a n joining a with the node, n. Similarly the 

 image of b will be formed at 6'. If a and b still remaining the same 

 distance apart, be moved nearer the eye to c and d, then the 

 visual angle under which they are seen will be greater and their 

 retinal images will be farther apart, at c' and d'. If a and 6 are 



