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
Dioptries of the Eye 
[125, 126. 
same index of refraction as the aqueous humor. The focus of the 
central rays is designated by F. The outer rays do not go through 
this focus, but intersect the adjacent rays and form thus a caustic 
surface, a meridian section of which is represented by the arc GF. 
The outermost ray CB is refracted along BH. The caustic curve GF 
ends at the point G at the middle of the chord in which the straight 
line BH cuts the circle. Suppose now that planes are passed through 
the refracting sphere situated like the iris in the aqueous humor. For 
example, if the straight line qoPo is the trace of a plane perpendicular 
to the plane of the diagram, its entire anterior surface would be 
illuminated with light. But if the plane were passed through qiPi, 
part of it would lie in front of the outside refracted ray BG, and be 
illuminated. The part beyond it would remain dark. If the plane 
were passed through Ç2-P2, it would cut the caustic surface. Here 
again a part would be light and a part dark, but the boundary between 
the illuminated and non-illuminated portions would stand out as a 
bright line corresponding to the line in which the plane q^P^ cuts the 
caustic surface. It may be seen from the figure that, if the portion of 
the plane, q^P^ which cuts the caustic surface, were to move back¬ 
wards, that is, away from the refracting surface, the bright line would 
approach the border. 
Now this can be observed on the iris when the eye is accommodated 
for near vision. If the patient’s eye is illuminated from the side so 
that the caustic curve appears at the ciliary margin of the iris, and if 
he looks alternately at a near and a far object in the same line, the 
caustic curve will be seen to approach ’the 
margin of the iris on accommodation for 
near vision and move away from it in far 
vision. This illumination of the iris is shown 
£ in Fig. 65. The light falls on the eye from 
the side in the direction of the arrow. The 
corneal reflection of the light is visible at b 
on the side towards the light. The caustic 
curve with its light shining partly through 
the projecting edge of the sclerotica is seen on 
the other side at a. 
According to the suppositions of Cramer and Donders, the iris 
and the ciliary muscle produce the change of the form of the lens by 
increasing the pressure in the vitreous body and on the outer margin 
of the lens in such fashion that only the centre of its anterior surface 
behind the pupil is free from the increased pressure. It is possible 
that the increase in convexity of the anterior surface of the lens which 
Cramer noticed first might be explained on this basis.


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