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THE POPULAR SCIENCE MONTHLY. 



by the alert ocular muscles. "When, however, the object is too 

 near the eye, or when its motion is too swift, the muscles are not 

 quick enough in their action to preserve this delicate state of op- 

 tical rest ; the image is thrown across the retina, and the object is 

 seen to move. A glance at this diagram (Fig. 3) will show how 

 these retinal impressions are received and interpreted. 



The first figure is intended to represent three objects seen from 



a train in motion. Although the middle one is fixed by the eye, 



^ and is consequently most dis- 



\ 



4^ 



tinctly seen, the blurred images 

 of the other two also fall upon 

 the macula, so that for a single 

 instant they are all optically at 

 rest. A moment later, the eye, 

 still fixing the middle object, 

 has moved from 1 to 2, and, as 

 is seen in 2, the images corre- 

 sponding to the near and the re- 

 mote objects have passed over 

 the retinal area. Motion in the 

 opposite direction is, according 

 to the law just laid down, at- 

 tributed to each, while the mid- 

 dle object still gives the im- 

 pression of comparative rest. 



When, however, the image 

 of the moving object is kept 

 fixed upon the macular region, 

 the eye may judge of the rate 

 of its motion by the amount 

 of effort put forth by the ocu- 

 lar muscles necessary to keep the image focused upon the macula. 

 This method of calculation is defective, and gives rise to nu- 

 merous optical errors. For example, the movement of a lady's fan 

 in front of her face, the velocity of a base-ball through the air five 

 hundred yards off, and the rate at which the night express trav- 

 els as it approaches " end on " — its head-light gleaming in the dis- 

 tance — would all be incorrectly calculated if the brain were to 

 accept ocular evidence alone and based on one or both of the fore- 

 going rules. The to-and-fro movement of the fan would be inter- 

 preted as exceedingly quick ; the velocity of the base-ball would 

 be next in order ; while little or no motion would be attributed to 

 the approaching train. 



Becoming tired of looking at the wayside scenery, I find my- 

 self, in a sort of brown study, watching the back of the plush- 

 covered seat in front of me, and then I discover that the retinal 



Fig. 3. 



