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
Dioptrics of the Eye 
worked out with much success by Dimmer1 in 'photographing the fundus 
of the eye; and is used likewise in Thorner’s2 stationary ophthalmo¬ 
scope. In both methods an image of a stop in the observation system 
is formed in the entrance-pupil of the patient’s eye. But whereas in 
the first method an image of the source of light is formed in the other 
part of this pupil, in the latter method there is at this place an image 
of a stop that belongs to the illumination system. In Dimmer’s method 
the distance a is large enough to shut off also the diffusely reflected 
light, but this does not seem to be the case with Thorner’s ophthal¬ 
moscope, so far as the crystalline lens is concerned. However, here this 
light is not so harmful, because the part of the pupil affected by the 
illumination system is comparatively large. In Wolff’s3 method of 
ophthalmoscopy without reflex there is no image of a stop in the ob¬ 
servation system in the pupil of the patient’s eye, and the observation 
system is separated from the illumination system by the screening 
of the mirror. By their methods also both Thorner and Wolff have 
made photographs of the fundus of the eye. Wolff’s electrical oph¬ 
thalmoscope is intended for clinical use and is employed for investigat¬ 
ing with the ordinary erect image, but it has the disadvantage of 
requiring the pupil to have a certain diameter. 
All methods of ophthalmoscopy without reflex in which an opaque 
mirror is adjusted to one side give a one-sided shading off of the field, 
and are disadvantageous in this way. 
In Thorner’s4 latest method of ophthalmoscopy without reflex, 
he employs a means of simple ophthalmoscopy which is due orginally 
to Schulten,5 and in which the lens of the ophthalmoscope is replaced 
by a concave mirror belonging to both the observation system and the 
illumination system. Here only a small source of light is needed, with 
its image in the mirror formed near the entrance-pupil of the eye or 
of the telescopic lens, in order to fulfil the conditions of ophthalmoscopy 
without reflex. Besides some inconveniences of a technical nature, the 
disadvantages of the method, such as astigmatism and lack of sym¬ 
metry in the bundle of rays used in the imagery, are due to the un¬ 
avoidable obliquity of the mirror. A glass mirror silvered on the back 
gives double images, the fainter one perhaps being comparatively 
1 Fr. Dimmer, Die Photographie des Augenhintergrundes. Wiesbaden 1907. 
2 W. Thorner, Die Theorie des Augenspiegels und die Photographie des Augenhinter¬ 
grundes. Berlin 1903. 
3 H. Wolff, Zur Photographie des menschlichen Augenhintergrundes. Arch. f. Augen¬ 
heilkunde. LIX. S. 115. 1908. 
4 W. Thorner, Ein reflexloser Handaugenspiegel. Zft. f. Augenheilkunde. XXVI. 
S. 1. 1910. 
5 Schulten, Beobachtungen des Augenhintergrundes bei hochgradiger Vergrösserung. 
Arch. f. Anat. u. Physiol. 1883. S. 285.


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