Bauhaus-Universität Weimar

Titel:
Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Person:
Helmholtz, Hermann von
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39649/503/
G.] 
VI. Ophthalmoscopy 
479 
harmless usually, and yet for certain effects necessarily in the way; 
and other mirrors are too delicate to be used in practical ophthalmo¬ 
scopy. 
Stereoscopic ophthalmoscopy without reflex occupies an intermediate 
place between central and eccentrical (or acentral) ophthalmoscopy; 
because in order to obtain the maximum stereoscopic effect, the 
images of the two stops of the observation systems must be close to 
the ends of a diameter of the entrance-pupil of the patient’s eye ; which 
means that the axis of symmetry of the two systems shall be centrally 
situated. If these stops are the entrance-pupils of two telescopic lenses, 
and if their images are formed in the entrance-pupils of the two eyes 
of the observer, the condition of correct stereoscopic effect is, that the 
magnification-ratio for this imagery of the distance between the eyes 
shall have the same sign, as for the imagery of the pupils. Hence, the 
astronomical telescope cannot be used, but the telescopic lenses must 
be made on the order of the terrestrial or prism telescope, in which 
the image of the fundus as presented to the eyes is inverted. The image 
of the slit may be vertical and midway between the images of the two 
stops, or horizontal and above or below these images. Assuming that 
the magnification-ratio for the images of the entrance-pupils of the 
telescopic lenses in the entrance-pupil of the patient’s eye is —1/3, it is 
advisable to take the distance between the centres of the stops at 16 mm, 
the diameter of each stop being 6 mm. The distance might perhaps be 
made greater, but then the instrument could be usëd only for a pupil 
of maximum size. On the other hand, there is not much advantage in 
getting a greater stereoscopic effect than can be obtained in this way. 
The magnification K= — 1 may be used with advantage. 
Thorner’s stationary ophthalmoscope mentioned above was used 
also for stereoscopic ophthalmoscopy, by halving again the half of the 
pupil intended for the observation system. Aside from other drawbacks 
of this method, the maximum stereoscopic effect is not obtained in 
this arrangement. 
The apparatus for ophthalmoscopy without reflex is fairly elaborate, 
due principally to the separation of the observation system from the 
•illumination system, which is necessary in order to avoid the reflex 
images in the lens of the ophthalmoscope. But if these reflex images 
can be tolerated, the apparatus can be essentially simplified. As a 
matter of fact, with a small source of light, these images are not very 
much in the way. Besides, as distinguished from the images re¬ 
flected at the surfaces of the ocular media, they can be made to fall in 
front of any part of the fundus of the patient’s eye. Thus, provided 
the magnification is not excessive, the advantages of ophthalmoscopy 
without reflex can be obtained in this way. Consequently, those
        

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