THE EYE AS AN OPTICAL INSTRUMENT. 217 



under ordinary circumstances, and why it is in fact 

 somewhat less than a glass instrument of the same 

 construction would have, is that the chief refractive 

 medium of the eye is water, which possesses a less dis- 

 persive power than glass. 1 Hence it is that the chro- 

 matic aberration of the eye, though present, does not 

 materially affect vision with ordinary white illumination. 



A second defect which is of great importance in optical 

 instruments of high magnifying power is what is known 

 as spherical aberration. Spherical refracting surfaces 

 approximately unite the rays which proceed from a lumin- 

 ous point into a single focus, only when each ray falls 

 nearly perpendicularly upon the corresponding part of 

 the refracting surface. If all those rays which form the 

 centre of the image are to be exactly united, a lens with 

 other than spherical surfaces must be used, and this 

 cannot be made with sufficient mechanical perfection. 

 Now the eye has its refracting surfaces partly elliptical ; 

 and so here again the natural prejudice in its favour led 

 to the erroneous belief that spherical aberration was thus 

 prevented. But this was a still greater blunder. More 

 accurate investigation showed that much greater defects 

 than that of spherical aberration are present in the eye, 

 defects which are easily avoided with a little care in 

 making optical instruments, and compared with which 

 the amount of spherical aberration becomes very unim- 

 portant. The careful measurements of the curvature of the 

 cornea, first made by Senff of Dorpat, next, with a better 

 adapted instrument, the writer's ophthalmometer already 

 referred to, and afterwards carried out in numerous 

 cases by Bonders, Knapp, and others, have proved that 

 the cornea of most human eyes is not a perfectly sym- 



1 But still the diffraction in the eye is rather greater than an instrument 

 made with water would produce under the same conditions. 



