THE EYE AS AN OPTICAL INSTRUMENT. 215 



All these changes of direction and of accommodation 

 take place far more slowly in artificial instruments. A 

 photographic camera can never show near and distant 

 objects clearly at once, nor can the eye ; but the eye 

 shows them so rapidly one after another that most people, 

 who have not thought how they see, do not know that 

 there is any change at all. 



Let us now examine the optical properties of the eye 

 further* We will pass over the individual defects of 

 accommodation which have been already mentioned as 

 the cause of short and long sight. These defects appear 

 to be partly the result of our artificial way of life, partly 

 of the changes of old age. Elderly persons lose their 

 power of accommodation, and their range of clear vision 

 becomes confined within more or less narrow limits. To . 

 exceed these they must resort to the aid of glasses. 



But there is another quality which we expect of optical 

 instruments, namely, that they shall be free from disper- 

 sion — that they be achromatic. Dispersion of light de- 

 pends on the fact that the coloured rays which united 

 make up the white light of the sun are not refracted in 

 exactly the same degree by any transparent substance 

 known. Hence the size and position of the optical 

 images thrown by these differently coloured rays are not 

 ''^It^ the same ; they do not perfectly overlap each other 

 in the field of vision, and thus the white surface of the 

 image appears fringedwith a violet or orange, according 

 as the red or blue rays are broader. This of course takes 

 off so far from the sharpness of the outline. 



Many of my readers know what a curious part the 

 inquiry into the chromatic dispersion of the eye has 

 played in the invention of achromatic telescopes. It is 

 a celebrated instance of how a right conclusion may 

 sometimes be drawn from two false premisses. Newton 



