THE EYE AND VISION 



211 



While the image of the candle-flame still falls on the ground-glass screen, 

 insert one of the diaphragms between the watch-glass and the lens. The 

 image now is not so bright as before, since a large proportion of the light is 

 shut off, but it is sharper ; or, in other words, the definition is better. Also, 

 by using diaphragms with different apertures, we find that, the smaller the 



Fig. 199. 



aperture, the sharper the image. Thus we have illustrated the use of the 

 iris the diaphragm of the eye, in shutting off the extraneous light which 

 tends to blur the image produced. 



We may illustrate the formation of images in the eye by a more simple 

 apparatus than that just described : Procure a globular glass flask to 

 represent the eyeball, and attach to the side of it a circular and perforated 

 piece of opaque paper to represent the iris (fig. 200). Now fix a convex lens 

 on a cork, and a piece of ground glass on another cork. Place the convex 

 lens in front of the iris, and the ground-glass screen a few inches from the 

 flask on the opposite side. The lens, of course, represents the crystalline lens, 



Fig. 200. 



and produces the same effect as it it were behind the iris ; and a clear inverted 

 image of a candle-flame may be formed on the ground glass by adjusting the 

 distance as in our previous experiments. 



As our last illustration, we may compare the eye as an optical instrument 

 to the photographer's camera. This consists of a dark box fitted with a con- 

 vex lens in front and a ground-glass screen behind. Rays of light from illu- 

 minated objects in front of the camera first pass through a diaphragm which 

 cuts off the outer rays. The central rays then pass on to the lens, by which 

 they are made to converge and form inverted images on the screen. This 

 screen is movable, so that it may be adjusted to receive the images of objects 

 at different distances without changing the lens. 



In the eye the retina is the screen on which the images fall ; 

 but this screen is not moved forwards and backwards as in the 



