SENSE OF SIGHT. 205 



afford examples of extreme delicacy of this as well as of their other 

 senses. They are necessarily compelled to cultivate it more ; and, 

 lastly, the musician, by education, attains the perception of the nicest 

 shades of musical tones. The aptitude is laid in cerebral organiza- 

 tion, and is developed by the education of the instrument the ear 

 as well as of the encephalic or intellectual organ, without which, as we 

 have seen, no such appreciation could be accomplished. 



SENSE OF SIGHT OR VISION. 



The immediate function of the sense of sight is to give us the notion 

 of light and colours. Like the other senses, it is a modification of that 

 of touch, whether we regard the special irritant light as an emana- 

 tion from luminous bodies, or as the vibration of a subtile, ethereal 

 fluid, pervading all space. Under the latter theory it would most 

 strongly resemble the sense last considered. 



The pleasures and advantages derived by the mind through this 

 inlet, are of so signal a kind as to render the organ of vision a subject 

 of universal interest. Every one, who lays the slightest claims to a 

 general education, has made it more or less a subject of study, and 

 is not unfrequently better acquainted with its structure and properties 

 than the medical practitioner. Complicated as its organization may 

 seem, it is, in action, characterized by extreme simplicity ; yet, " in 

 its simplicity," as Dr. Arnott 1 has remarked, " so perfect, so unspeak- 

 ably perfect, that the searchers after tangible evidences of an all- wise 

 and good Creator have declared their willingness to be limited to it 

 alone, in the midst of millions, as their one triumphant proof." Into 

 this structure we shall inquire, so far as is necessary for our pur- 

 pose, after having described the general properties of light; and then 

 detail the mode in which its various functions are accomplished, and the 

 knowledge derived by the mind through its agency. 



The eye is the organ of vision. It varies materially in different 

 animals; in some, consisting of a simple capsule, with the final expan- 

 sion -of the nerve of sight distributed on its interior, and communi- 

 cating externally by means of the transparent cornea, which admits 

 the light. It is in this simple state that M. de Blainville 2 assimilates 

 it to a bulb of hair, modified for the new function it has to execute. 

 In man, and the upper classes of animals, the organ is much more com- 

 plicated in its structure; and in it we have a still clearer example of 

 the distinction between the physical, and the nervous or vital part of 

 the apparatus, than in any of the other organs of sense, the former 

 consisting of transparent tunics, and humours, which modify the light 

 according to the laws of optics ; the latter being a production or ex- 

 pansion of nervous structure, for the reception of the impression of 

 light, and for conveying such impression to the proper part of the 

 encephalon. There is, besides, attached to the organ, a number of 

 accessory parts or tutamina, which are more or less concerned in the 

 proper performance of the function. It will be necessary, therefore, to 



1 Elements of Physics, 2d Amer. edit., vol. ii. P. i. p. 1(51, Pliilad., 1836. 



2 De ('Organization ties Animaux, Paris, 1825. 



