24 A TYPICAL VERTEBRATE EYE: THE HUMAN 



If the soldiers had encountered the wheat head on instead of at an 

 angle, their line of march would not have been tilted. But their ranks 

 would have been closed up, and while moving through the wheat each 

 soldier would have been treading on the heels of the man in front of him. 

 Strictly speaking, this would be refraction also, for the same decrease in 

 wavelength occurs when the angle of incidence is other than 90° — 

 refraction is most accurately defined in terms not of any bending of 

 the light rays, but of their change in speed and wavelength. Thus it 

 actually takes place when light meets a surface at right angles; but since 

 no visible change then occurs, the existence of the phenomenon is more 

 or less ignored. 



Substituting now our beam of light and piece of glass for the soldiers 

 and the wheat-field, we can understand why the angle at which the light 

 meets the glass is so important in determining the direction the beam 

 will take through the glass. If the angle be changed, the new direction 

 will change. If a perpendicular be drawn to the surface of the glass, then 

 the beam of light on entering the glass from air will be bent toward the 

 perpendicular; and upon escaping from the glass into air again it will be 

 bent away from a perpendicular at the point of escape, the two bends in 

 the beam being equal if the two surfaces of the glass are parallel. 



Action of a Convex Lens — We are now ready to understand how a 

 lens brings rays of light to a focus (study Fig. 9) . If a beam of parallel 

 rays of light strikes a convex lens, each ray in the beam will make an 

 angle with a tangent to the lens at the point where the ray strikes it, and 

 the angle will vary with the distance of the ray from the central ray of 

 the beam, which we will suppose to pass through the center of curvature 

 of the lens surface. The farther a ray is from the axial (central) ray, 

 the greater the angle it makes with a radius of the lens at its point of 

 contact with the latter, and the greater the angle of bending, toward 

 the radius, through which it will be refracted by the glass of the lens 

 (Fig. 9g). 



Thus, the outermost rays of the beam are bent the most, rays lying 

 closer and closer to the axial ray are bent less and less, and the axial 

 ray is not bent at all. All the rays thus converge beyond the lens and 

 if the shape of the lens surface is just right, they may be made to con- 

 verge at a single point. This point, or 'focus', will be at a fixed distance 

 from the lens, and that distance can be varied only in two possible ways 

 — by making the lens variable in curvature or by exchanging it for a 



