8o VISION. 



preciation of a picture does not appear at once in 

 the simpler forms of eyes met with in animals low 

 in the scale ; and perhaps the most interesting way 

 of looking at the structure of the human eye is to 

 compare it with others of a less complex sort. 

 The simplest form of indubitable eye is a nerve- 

 termination with pigment on it, and more simple 

 still there are spots of pigment in various animals 

 in which nerves are not to be found ; and those 

 spots are possibly or in some instances even pro- 

 bably eyes ; that is to say sensitive to the rays 

 which the pigment reflects. 



The star fishes furnish examples of a simple form 

 of indubitable eyes. They have not all got eyes ; 

 but those which possess them have them at the tips 

 of their arms in groups which form a scarlet spot. 

 We know that these organs are eyes because they 

 have not only bright colour round them, but have a 

 transparent structure at the extremity, which admits 

 light to the 'nerve-extremities behind it. There can 

 be no doubt, therefore, that light is the irritant 

 which acts on these organs ; but it is equally certain 

 that they are utterly incapable of displaying the 

 forms of surrounding objects, for every ray which 

 enters must fall on the red cup which surrounds the 

 nerve terminations, and in consequence of this its 

 red elements will be reflected from place to place in 



