PROFESSOR AT KONIGSBERG 135 



of the cornea, the alterations of the iris in accommoda- 

 tion, and lastly the curvature of the anterior and posterior 

 surfaces of the lens, which he determined with astonishing 

 perspicacity. 



Starting from the presumption that a convex mirror-surface 

 gives smaller images of the surrounding objects in proportion 

 as its radius of curvature is smaller, so that the radius of 

 curvature may be calculated from the size of the images, he 

 attempts to measure the size of the minute image on the 

 cornea, but is at once pulled up by the difficulty that the living 

 eye cannot be fixed as immovably as such an exact measure- 

 ment requires. In order to measure the free corneal image, 

 while the eye itself is in motion, he therefore applied the 

 principle of the heliometer (by which astronomers can estimate 

 the least distances of the stars in the moving heavens, notwith- 

 standing their apparent motions, so exactly that they can plumb 

 the profundities of the firmament of the fixed stars), applying 

 it in an altered form to the moving eye. He constructed the 

 ophthalmometer, by which he succeeded in measuring the 

 curvature of the cornea and other phenomena of the living 

 eye with greater accuracy than had hitherto been possible on 

 the dead eye. The principle of the ophthalmometer, which was 

 to play so great a part in physiological optics, depends on 

 the fact that objects observed through a glass plate with per- 

 fectly even and parallel surfaces, held obliquely to the line 

 of vision, seem to be displaced laterally, and that this dis- 

 placement increases with the increasing angle of incidence of 

 the rays of light upon the plate. When two plane-parallel 

 glass plates are rotated in opposite directions in front of a 

 telescope obliquely to its axis, two images of any object that 

 is within the field of the telescope appear simultaneously: if 

 the two glass plates are then rotated till the two images overlap, 

 Helmholtz showed that the size of the object observed can 

 be estimated from the magnitude of the angle of rotation, irre- 

 spective of the distance of the object from the telescope, because 

 the ophthalmometer shows the same linear displacement at all 

 distances. The limitations of the Institute compelled Helmholtz 

 to construct his telescope from materials which he happened to 

 possess, and the entire instrument, except the plane-parallel 

 glass plates, was made in Konigsberg ; but he soon suggested 



