2i4 PHYSIOLOGY FOR BEGINNERS CHAP. 



to be used. The reason why the short-sighted eye brings 

 the rays to a focus in front of the retina is usually not because 

 the lens is naturally more convex, but because the retina is 

 farther behind the lens than in the ordinary eye, the eye- 

 ball being longer. On the other hand, in a long-sighted 

 person the eyeball is shorter than usual and the retina lies 

 too near the lens. He sees distant objects very well, but 

 the rays from near objects are brought to a focus behind 

 the retina, and so the image on the retina is indistinct. 

 Convex spectacles have to be used. Old persons do not see 

 clearly near objects for a different reason. In them the power 

 of accommodation for near objects has more or less failed. 

 Either the ciliary muscle does not contract so well, or the 

 elasticity of the lens is impaired, so that when they look at 

 near objects they cannot make the lens convex enough. 

 Convex spectacles have to be used. 



Action of the Iris. If, when as clear an image as 

 possible has been obtained in the simple experiment with the 

 glass lens, a sheet of thick black paper with a hole in it be 

 held immediately in front of the lens, a sharper, but less bright 

 image will be obtained on the screen. The rays that pass 

 through the outer part of a lens are brought to a focus a little 

 in front of those that pass through the more central part, 

 so that the image is not perfectly clear. By the use of 

 the paper with the hole some of these outer rays are cut off. 

 When the light is a long- way from the lens, the hole in the 

 paper will have to be a large one, in order to get the best 

 image, as regards clearness and brightness taken together ; or 

 even letting the light come through the whole lens may give 

 the best result. On the other hand, when the light is near the 

 lens, a paper with a smaller hole will give the best result, 

 because in this case more light is falling on the lens, and so 

 more of the peripheral part can be cut off without making the 

 image too dim. A photographer uses the " stops " he puts in 

 front of the lens of his camera on the same principle, and he 

 obtains inverted images on his sensitive plate in the same way 

 as is done in this simple experiment. The iris in the eye acts 

 in the same way. It cuts off the rays which would otherwise 

 pass through the outer parts of the crystalline lens and be 

 brought to a focus too soon. 



