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phibians, while in fishes they are the shortest. In birds the diameter of the cones 

 approaches very closely to that of the reptiles. 



When one is comparing the sensitiveness of the retina of difTerent animals 

 the diameter of the rods and cones is of vast importance. For since these sensi- 

 tive elements are arranged close together, where the diameter is small there would 

 be more per given area and a more sensitive retina. 



The relative number of rods and cones is also of importance. In mammals 

 and amphibians the rods far surpass the cones in number. In birds, with few ex- 

 ceptions, the reverse is true, while in reptiles few or no rods are present. In 

 fishes they are more equally divided. 



Investigations by experiment and histological examination prove that the 

 rods are more sensitive to faint impressions than the cones, but that the cones 

 have the greatest power of discrimination both of color and shade. Most noc- 

 turnal animals that liave been examined have few or no cones. 



Experiments on the human retina show that the fovea has the power of 

 most acute vision, and that the power of distinct vision grows rapidly less toward 

 the periphery. We may thus assume that in other animals the fovea, which has 

 the same general arrangement of retinal elements as in man, when present bears 

 the same relation to the more peripheral parts. The human macula, though 

 inferior to the fovea, sees objects more distinctly than the peripheral parts, and 

 we may reasonably say that in general the area centralis bears this relation to 

 the other parts of the retina. 



The peripheral part serves as a sentinel, for it perceives objects in motion 

 more easily than objects at rest. Moving objects attract all animals more quickly 

 than stationary ones, and this is especially true in those animals whose retinal 

 development has not proceeded beyond the differentiation of an area. Only those 

 animals which possess a fovea seem to have the power of quiet and close dis- 

 crimination of an object at rest. 



In speaking of the powers of sight in the difTerent classes of vertebrates, I 

 can do no better than quote from the original article of which the foregoing is a 

 summary. 



Fishes as a rule depend upon sight for their food, excepting such as the 

 shark, which depends almost wholly on its smell. This class of vertebrates does 

 not, however, usually possess a fovea. 



How distinctly they see we can not say, but we know that the trout quickly 

 takes the fly when thrown on the water, or the pickerel the Avhirling spoon as it is 

 drawn before it. They see the objects while in motion and are apparently una- 

 ble to distinguish them from the real article of food. An experience in fishing 



