136 HERMANN VON HELMHOLTZ 



to Bonders a more practical construction, in order to obtain the 

 maximum of brilliancy in the images. 



When this instrument is used to measure the curvature of 

 the cornea, the latter must reflect the image of some external 

 object of known size and distance, and the magnitude of the 

 image can then be measured in the ophthalmometer. Helmholtz 

 made the important discovery that in all diseases of the eye 

 that are associated with alterations of pressure in the fluid 

 media, these changes can be detected on the cornea. His 

 measurements of the radius of curvature at different points of 

 the cornea showed that it corresponds in form with an ellip- 

 soid, produced by the revolution of an ellipse about its major 

 axis, so that the base of the cornea forms a plane vertical to 

 the major axis of the ellipse, and the central point of the cornea 

 coincides with the vertex of the ellipse. During accommodation 

 there is not the slightest alteration of curvature in the cornea. 

 This method could not be employed in determining the form 

 of the inner surface of the cornea, because the image from the 

 anterior corneal surface is so much stronger than that from 

 the posterior that the latter cannot be observed, if the two 

 occur very close together. Experiments with the cornea of 

 dead eyes showed, however, that the thickness of the cornea 

 scarcely alters at all in the two central quarters, but increases 

 fairly rapidly towards the margin. From this it may be 

 assumed, in calculating refraction of the eye, that the aqueous 

 humour extends to the anterior surface of the cornea. And 

 since the lens extends close to the iris, it is only necessary, 

 in order to determine the distance of lens from cornea, to 

 measure that of the pupillar border of the iris from the cornea, 

 which Helmholtz again accomplished by means of the ophthal- 

 mometer. It was also shown by a series of extremely delicate 

 observations that since the lens is always situated close to 

 the pupillar border of the iris, while the form of the cornea and 

 the volume of the aqueous humour are not altered in accommo- 

 dation, displacement of the middle portion of the iris and lens 

 cannot occur without such a backward movement of the iris 

 at its periphery, that the anterior chamber gains as much in 

 volume there as it loses in the centre. 



The curvature of the anterior part of the lens cannot be 

 measured directly by the images, because the reflected image 



