470
Dioptrics of the Eye
[G.
though both the field and the intensity are very small, in many cases
it is possible in this way to see the small reflex from the fovea, when it
is not possible to do so in the ordinary investigation. However, a
sufficient intensity can be obtained with the Nernst slit lamp, which
the writer designed originally for use in ophthalmometry, and which is
consequently known also as the Ophthalmometrie Nernst lamp.
It consists of a closed tube, at one end of which the lamp is inserted; a
lens-system being used to form an image of the little incandescent
filament in a slit at the other end of the tube, so that this illuminated
slit is to be regarded as the source of light. With a vertical slit a bright
enough image is obtained by this arrangement, and in most cases the
macula region can be investigated without dilating the pupil. Of
course, the field becomes smaller as the pupil is more contracted; but
even when a pupil is contracted by treatment with eserin, it is possible
by this method to see a small field in the fovea. When the distance
between slit and mirror is too great, the condition as to the size of the
image of the source of light is not satisfied ; but a central dark spot will
be seen in the illuminated field, which moves in conformity with the
movements of the mirror. However, by using a concave mirror with
a radius of about 15 cm, a slit from 12 to 15 cm away from it, and an
angle of incidence of approximately 45°, this spot will usually disappear.
The reason why this is so with a source of light almost linear in form
is due to the fact that the bundle of rays after being reflected is not only
astigmatic, but has a fairly high asymmetry-value. With this adjust¬
ment, after refraction of the light into the optical system of the eye, the
second focal point falls on the retina or very near it, provided the re¬
fraction of the eye is very anomalous; and owing to the asymmetry
the width of the bundle of rays at the first focal point is finite. If this
investigation is performed with dilated pupil, it constitutes a diag¬
nostical method which is more delicate than any previous method of
this sort. The illuminated field here is very bright and large enough to
be investigated conveniently. Besides, owing to the abolition of the
corneal reflex, the investigator is enabled to use the central part of the
pupil of the patient’s eye in the observation system; which would
otherwise be impossible, because the corneal reflex here compels the
observer to make a movement to one side. Hence, in the ordinary
investigation of the macula region by the method of erect image, the
observer is always forced to use the more unfavourable part of the
pupil for the optical imagery in the observation system; whereas with
this method of simple central ophthalmoscopy the resolving power of the
optical system of the eye is essentially greater, thereby permitting
much finer details to be perceived.