OPTICAL PRINCIPLES OF THE EYE.' 381 



the impression of light, or rather of rendering it more intense by central- 

 izing it upon a special locality. Such a construction involves at once a 

 change in the nervous mechanism, by devoting a particular system of 

 nerve tubules to the new duty. But, notwithstanding this increasing 

 complexity of structure, the physical principle is still as simple as be- 

 fore. It is indeed almost as though a blind man should paint upon his 

 skin a black space, so that, as in Franklin's experiment, it might be 

 more sensitive to the sun. With this devotion to a new duty the nerv- 

 ous tubules doubtless assume an isolated function, and thus there arises 

 a nerve of special sense. The ocelli of the lower animals are sometimes 

 quite numerous. Fronl this a new power is at once derived, the power 

 of determining the position of the source of light, a property which doubt- 

 less becomes more perfectly marked in proportion to the number and 

 symmetry of arrangement of the ocelli. As we ascend the animal series 

 in our examination, we soon find that complexity is being introduced. 

 A membranous hood, arising from a little fold of the external tegument, 

 shadows forth the rudiment of an eyelid, and seems to indicate to us that, 

 even in these low grades, the condition which we shall eventually find 

 so strikingly marked in the high ones already exists, that functional ac- 

 tivity involves destruction, and that the sensory mechanism must have 

 its period of repose. 



Approaching the more highly-developed conditions of the organ of 

 vision, we may next consider the cases presented by the i ntroduction of 

 eyes of insects and the eyes of higher mammalia. In these converging me- 

 a new physical principle has been introduced, the optical ia ' 

 property of the convex lens, a transparent solid, having one or both of 

 its surfaces curved, and obtaining therefrom the power of forming repre- 

 sentations, or images of objects which may be in front of it, at a certain 

 focal distance behind. Such images are seen when we take a magnify- 

 ing glass or a convex lens, and, holding a piece of paper be- The focus and 

 hind it at a particular point, there will be depicted upon the its variations 

 paper the inverted forms of whatever objects may be in front. 

 That distance is the focal length of the lens. But we may farther no- 

 tice, and to this observation our attention will be required hereafter, that 

 the focal length is variable. If the object be near, the focal length is 

 greater ; if distant, it is less. 



The instrument known as the camera obscura represents the optical 

 construction of the eye. Upon a receiving surface or screen, The camera 

 placed at the focal distance behind its lens, images are depicted obscura. 

 of whatever objects may chance to be in front ; but and this is a remark 

 of interest to us now the visual range, or field of view, is quite limited. 

 In animals, the perfection of whose vision requires that, instead of being 

 restricted, in their view to a narrow space, they should be able, as it 



