SECTION VI 

 THE REFRACTION OF THE EYE 



SINCE the eye forms an image of external objects by means of its refracting 

 media, it is found to have properties and to suffer from defects similar to 

 those met with in the case of other optical systems. We may therefore 

 treat the eye as if it were an optical instrument and estimate its efficiency 

 from that point of view. In the first place therefore we must consider what 

 kind of an image it would form if it were a perfect lens system, suffering from 

 no kind of aberration. Our experience of other lens systems, well- 

 nigh perfect, has shown us that the image of a distant point source of light 

 is not a mathematical point, as geometrical optics would have us believe, 

 but is on the contrary a definite pattern of quite considerable size, the shape 

 and dimensions of which can either be calculated from the conditions, or 

 may be seen, measured and photographed by appropriate means. The 

 formation of this pattern is due not to any defect of the lens, but to the 

 fact that light is a form of wave motion, which exhibits a property called 

 diffraction. 



DIFFRACTION OF LIGHT. When a broad wave of light surges through 

 the ether from a distant source of light, its front travels straight and inflexible 

 under the influence of certain well-known laws. These in popular language 

 restrain the tendency of a portion of the wave front to deviate, because of 

 an equal effort of a neighbouring portion of the wave front to do the same 

 thing, only in the opposite direction. At the edges of the wave on the other 

 hand there is no neighbouring portion from which to obtain support, and 

 therefore these edge portions tend to spread sideways more and more from 

 the main wave. A narrow ray of light therefore after passing through the 

 pupil of an optical system, will show this phenomenon to such a marked extent 

 that only a small portion of the total amount of light will actually reach 

 the goal defined by the original wave front. The smaller the pupil and 

 therefore the narrower the beam of light, the greater the amount of spreading 

 ;hat we should expect to find. And experiment shows that this is the case. 

 Further, we should suppose that in the case of any one beam the more spread- 

 ng would occur the further the ray has to travel. This also is found to 

 3e the case. Lastly if waves of different wavelength were tried, we would 

 < expect a short wave to suffer more than a long, because to the short wave the 

 distance travelled would seem relatively greater. And so it is. We can 

 therefore summarise the above by saying that the size of the diffraction 

 pattern formed by any lens system varies directly as the focal length of the 

 tystem and the wavelength of the light, and inversely as the diameter of 



529 34 



