SIGHT 141 



moved towards or away from the screen in focusing, as in 

 the fishes. In fishes focusing is done by moving the lens to 

 and fro, and not by altering its curvature. There are limits, 

 however; when the muscle fibres are relaxed the lens is 

 focused on infinity and all distant objects are in focus. The 

 muscles contract to keep an object in focus as it comes nearer 

 but when the sling is quite slack the curvature of the lens 

 can increase no more and the object then goes out of focus. 

 The normal distance at which this accommodation for near 

 objects is comfortable for us is about ten or eleven inches — 

 the usual reading distance — but by making a special effort 

 things can be focused nearer to the eye for a short time 

 although this action soon fatigues the muscles. If we want 

 to look at things so small that they must be brought closer 

 to the eye than the distance at which we can accommodate 

 we have to help the eye with a hand-lens or the combination 

 of lenses in a microscope. 



As we grow old the elasticity of the lens decreases so that 

 it becomes more and more difficult for the muscle fibres to 

 alter its shape to accommodate for near vision — we get 

 "long-sighted" and have to help our sight by wearing 

 spectacles for seeing near objects or reading. "There's noth- 

 ing wrong with my eyes," the patient said to his doctor, "but 

 my arms are too short to hold the book far enough away to 

 read the print." On the other hand, people who are short- 

 sighted in youth find they can discard their spectacles when 

 the increasing long-sightedness of more mature years brings 

 compensation. 



The edges of a simple lens focus the light slightly in front 

 of the image given by the central part, so that the picture 

 is a little blurred. The sharpness of the picture is increased 

 if we prevent this by using only the centre of the lens, but 

 of course its brightness is diminished for less light comes 

 through. In a camera the edge of the lens is blanked out by 

 using a diaphragm, and in the eye the coloured iris serves 

 the same purpose. The muscles in the iris make the pupil in 

 the centre larger or smaller according to the brightness of 

 the light and thus there is a compromise between sharpness 

 and brightness — when the light is poor the eye has to put 

 up with a slightly fuzzy image. 



