Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
G. 361, 362.] 
V. Monochromatic Aberrations of the Eye 
is nothing that can be responsible for this idiosyncrasy of the wave 
surface except a similar peculiarity of the refracting surfaces of the 
crystalline lens or of the iso-indicial surfaces, because the star-figure 
around a luminous point disappears when the lens is removed from the 
eye. This form ought to be manifested in the surfaces of the lens by 
jerky movements of the reflex images during movements of the eye. 
As a matter of fact such twitchings are often perceived in the reflexes 
in the anterior surface of the lens. But so far as the writer has been 
able to discover, this phenomenon occurs only at the periphery of the 
lens, and this would not be sufficient for the explanation of the 
“creases.” Besides, the reflex image in question is produced not simply 
by the anterior surface of the lens, but also in the most anterior portions 
of the lens substance. This is why it is so vague. The only other hy¬ 
pothesis left, is that the forms of the iso-indicial surfaces of the lens 
correspond to that of the wave surface. Taking into account the 
anatomical structure of the lens, we reach the same conclusion from 
the study of the dioptrics of the lens. During the accommodative 
change of form of the lens, the iso-indicial surfaces must contain 
constant volumes; and hence for the different focusings of the eye 
their superficial areas must be different, supposing they are surfaces 
of revolution. But this would not be possible unless either the particles 
of the lens were perfectly mobile, or the substance of the lens had con¬ 
siderable elasticity. Now since neither of these is the fact, the different 
forms assumed by the iso-indicial surfaces cannot be surfaces of revolu¬ 
tion; which means that the change of form must be accompanied by 
the production of “creases” or by the variation of such “creases.” 
Owing to the concomitant contraction of the pupil in accommodation, 
a variation such as that just mentioned is difficult to investigate in a 
perfectly satisfactory manner. However, in case of accommodation 
that occurs in the first stage of the action of eserin on a pupil that is 
dilated by homatropin, it is easy to verify the fact that there is some 
change of this nature. 
The fact that the star-figure is due to the iso-indicial surfaces of the 
lens and their variations during accommodation, plainly shows that 
this particular peculiarity of these surfaces is influenced by the tension 
of the zonule. That the effect cannot depend on the anatomical struc¬ 
ture of the lens, which in its embryonic stage has the shape of a three- 
point star, is shown both by the number and by the disposition of these 
points; whereas in the most regular cases the star-image itself consists 
of eight rays, its fundamental form being that of a cross with diagonal 
rays. On the other hand, in the mechanism by which the lens is sus¬ 
pended there is an anatomical contrivance that must produce alternate 
maximum and minimum tensions of the zonule in the different meridian


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