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