374? Mr. Addams on a peculiar Optical Phenomenon. 



transverse plane passing through a vertically moving body, 

 they will naturally and even irresistibly tend to follow the 

 motion of that body; nor can the muscular apparatus of the 

 eye maintain a stable equilibrium when the sight is fatigued 

 and bewildered with a rapid change of moving forms before 

 the eye. 



Now in the case of the descending water, the eyes, being 

 directed to a particular part in a horizontal section of it, can- 

 not be prevented moving downwards through a small space : 

 every new form in the moving scene invites the eyes to ob- 

 serve, and for that reason to follow it; but the voluntary 

 powers are engaged to raise the axes of the eyes again to the 

 section. This depression of the axes below the intentional 

 point of sight seems to be repeated three or four times per 

 second, whilst looking at the water-fall. Then, when the eyes 

 are suddenly turned upon the rock, the muscles, having been 

 brought into a kind of periodic contraction, will perform at 

 least one of these movements after the exciting cause ceases 

 to act; and thus the axes of the eyes, by moving downwards, 

 will occasion a motion of the image of the rock over the re- 

 tina in a direction from above downwards, and consequently 

 the object giving that image will appear to move the con- 

 trary way, that is, upwards, agreeably to observation. 



The deception, so far as I could judge, seemed to continue 

 for a time equal to the interval of a periodic motion of the 

 eye downwards when looking at the water, and, as before 

 stated, one third or one fourth of a second. 



The same kind of phenomenon may be produced by mov- 

 ing the eye before fixed bodies, and also when the motions 

 are executed horizontally. 



I have since been enabled to observe the appearance, with 

 certain peculiar variations, whilst travelling parallel to one 

 side of a narrow valley or lake, and looking across to the 

 other. It takes place when moving in ships in sight of proxi- 

 mate land. 



It is also producible by mechanical means, such as by a rapid 

 unrolling of pieces of calico having some pattern or markings 

 on them ; and likewise by moving the head up and down, or 

 laterally: but to particularize all the circumstances would 

 make this communication inconveniently long. 



