THE EYE AS AN OPTICAL INSTRUMENT. 191 



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 dispersion 

 that they be achromatic. Dispersion of light depends 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 quite the same : they do not perfectly overlap each other 

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

 appears fringed with 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 premises. Newton thought he had discovered a relation 

 between the refractive and dispersive powers of various trans- 

 parent materials, from wtiich it followed that no achromatic 

 refraction was possible. Euler, 1 on the other hand, concluded 

 that, since the eye is achromatic, the relation discovered by 

 Newton could not be correct. Reasoning from this assumption, 

 he constructed theoretical rules for making achromatic instru- 

 ments, and Dollancl 2 carrisd them out. But Dolland himself 



1 Leonard Euler born at Basel, 1707 ; died at St. Petersburg, 1783. 



2 John Dolland, F.R.S., born 1706; died in London, 17G1. 



