FtTNCTIONS OF THE VARIOUS EYE STRTJCTTTEES 459 



backward movement. If, now, we place this box with the 

 aperture pointing toward an illuminated object and move 

 the lens backward or forward, we find that for a certain 

 position of the lens there appears on the groimd glass 

 screen an inverted picture of that object. In other words, 

 this combination of lens, screen, and aperture, with its 

 light-excluding walls enables us to project inverted images 

 of external objects upon the screen. If we should replace 

 the screen in this apparatus by a sensitive photographic 

 plate we should obtain a permanent impression of the 

 image or picture. Our box would then fulfil aU the func- 

 tions of a camera. 



Now, the human eye is essentially such a sort of image 

 producer as this box or camera. The only difference is 

 that, instead of a sensitive photographic plate capable of 

 one exposure and producing one picture, we have a ner- 

 vous structure, the retina, which receives these impressions 

 and transmits them to the brain as nerve impulses. This 

 retina is always ready to receive impressions; the human 

 camera is always " loaded." But the resemblance to the 

 camera does not stop with the retina. The pupil of the 

 eye with its iris serves to regulate the amoimt of light 

 which shall enter, just as the cardboard disks or stops of 

 a camera do. The sclerotic layer corresponds to the 

 wooden or metal walls of the box, and like those gives shape 

 and strength to the structure.- The black, inner choroid 

 coat, by means of its pigment, absorbs light, and prevents 

 internal reflection, just as does the black paint on the 

 inside of the camera box. Finally, the cornea and lens cor- 

 respond to the lens of the camera, and, like that, produce 

 inverted images upon the sensitive surface or retina. 



The points of difference between the eye and a camera 



