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/lit39649/494/
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.
        

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