586 



STUDIES IN ADVAXCKD PHYSIOLOGY. 



ing point on the retina ;/ n at c. If the focussing were not 

 distinct and the rays of light should strike the retina as they 



Fig. 184. DIAGRAM TO SHOW THK EFFECTS OF IMPERFECT FOCUSSING IN PRODUCING 



THE PHENOMENA OF IRRADIATION. 



At either m, m, or I, I, there is a circular field of light instead of a point as at , n. 



do at m m or at / /, a much larger portion of the retina would 

 be covered. The image instead of being a point would be 

 a disk. This disk covering a larger portion of the retina 

 than it would do if it were exactly focussed, produces a sen- 

 sation which is interpreted as much larger in extent. Thus, 

 also, a black area in a light background would seem larger 

 than it naturally is, because now the black area would not 

 be sharply focussed, would spread over more retina and give 

 rise to a sensation of larger extent. For this reason a per- 

 son in dark attire would seem larger when projected against 

 a white background just as a person in light attire seems 

 larger when projected against the usual dark background. 



2. Parallelism. While we are usually able to distin- 

 guish with considerable accuracy the parallelism of two 

 lines, there arises a deception when these lines are crossed 

 with diagonal lines, as indicated in Figure 185, where the 

 long lines are parallel, but seem not to be. 



3. Distance. We judge distance in several ways; by 

 the size which the object appears to us to have when its 

 actual size is known ; by the distinctness with which we see 

 it, and by the number of objects which lie between us and 

 it. Unbroken distances are as a rule entirely under-judged. 

 This is due to the fact that distances in which many objects 

 are found seem longer than distances in which no objects 



