478
Dioptrics of the Eye
[G.
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.