PROFESSOR AT KONIGSBERG 137 



is not sharp. The size of the image has to be compared with 

 that of a corneal image close to it, by means of two reflected 

 objects, one of which must be of varying magnitude, in order 

 that the corneal image of one may be made equal to the Sanson's 

 image of the other. It was then ascertained by means of the 

 ophthalmometer that in near accommodation the anterior surface 

 of the lens is more strongly curved, the radius of curvature 

 is accordingly smaller, and its vertex is pushed forward. When 

 Helmholtz applied the method to the posterior surface of the 

 lens, he discovered, in determining the position of the latter, 

 and the question whether cornea and crystalline lens are 

 symmetrical to the same axis, that, in the eyes under examina- 

 tion, there was a slight but perceptible defect of centring, 

 which produced the so-called astigmatism of the eye : the 

 effect of which is that we cannot clearly see horizontal and 

 vertical lines at the same distance simultaneously. This he 

 characteristically expresses by saying that the eye, in spite of 

 its wonderful powers, is an instrument so full of serious defects 

 that if a mechanician turned out anything so imperfect he 

 would show him to the door. In regard to the actual curvature 

 of the posterior surface of the lens, he found that it became 

 a little more convex during accommodation, and did not alter 

 its position perceptibly. Lastly, in regard to the question how 

 the observed changes of form in the lens are produced, he 

 inclines to the view that the ciliary structures must be admitted 

 to participate in some way or other in the movements of 

 accommodation. 



Writing before the publication of this work, Helmholtz 

 informs du Bois that the article on Accommodation in 

 Graefe's Archiv fur Ophthalmologie is in the press, but not 

 out yet: 



' I have determined the measurements of the curvature of 

 the cornea and anterior and posterior surfaces of the lens, and 

 their distances in the living eye, by new methods not indeed 

 with the greatest attainable accuracy, but only so as to 

 show people that it can be done ; for I realized during the under- 

 taking that it would be useless to expend great pains upon 

 it. The human eye is not even properly centred, the magni- 

 tude of the corneal excentricity appears to be quite irregular 

 and adventitious, and so on. You must judge the paper from 





