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/lit39649/263/
205, 206.] 
§16. Illumination of the Eye and the Ophthalmoscope 
239 
bundle of rays proceeds from this point x to the pupil of the patient’s 
eye. According to propositions I and II, some of this light must 
return to the illuminating object, and some of it must go to the pupil 
of the observer’s eye. Suppose P denotes the area of the pupil of the 
patient’s eye, and p denotes the area of the cross section in this same 
plane of that part of the bundle of rays which returns to the illuminat¬ 
ing object. Moreover, let H denote the brightness of illumination 
that would exist at the given place on the retina if the patient looked 
directly at the source of light and focused it on his retina. This magni¬ 
tude may be called the normal brightness of illumination. It depends 
essentially on the structure of the retina itself, and, of course, also on 
the brightness of the illuminating object and on the size (P) of the 
pupil of the patient’s eye. When an ophthalmoscope is employed, the 
actual brightness of illumination of this part of the retina is necessarily 
less than this; that is, it is equal to 
Now let Q denote the area of the pupil of the observer’s eye; and let q 
denote the area of the cross section in this plane of the part of the 
bundle of rays coming from x that enters the pupil of the observer’s 
eye; then the brightness of this part of the retina as it looks to the 
observer is 
(b) Case when there is appreciable loss of light by reflection or 
refraction. In all the types of ophthalmoscope hitherto constructed 
the only one in which there is such a loss of light is in the case of the 
construction proposed by the author where unsilvered glass plates are 
used. In this case, and in all similar contrivances, the losses of the 
bundle of rays going from the eye to the illuminating object are pre¬ 
cisely the same as those of the rays that actually go from the light to 
the eye. Suppose that light of unit intensity proceeds from the source 
to the illuminated eye, and produces there an illumination a ; and that 
light of the same unit intensity proceeds from the patient’s eye to the 
observer’s eye and produces there an intensity ß: then the above 
expression has to be multiplied both by a and ß, so that it becomes: 
a-ß-V-q 
P'Q 
H. 
This complete reciprocity in the problem of the illumination of the eye as 
contained in the preceding propositions has enabled us to investigate the 
brightness of illumination of the images in every case by reducing the matter 
to the determination of the procedure of a single bundle of rays. Otherwise, 
the brightness at any particular place on the retina would have had to be found
        

Nutzerhinweis

Sehr geehrte Benutzer,

aufgrund der aktuellen Entwicklungen in der Webtechnologie, die im Goobi viewer verwendet wird, unterstützt die Software den von Ihnen verwendeten Browser nicht mehr.

Bitte benutzen Sie einen der folgenden Browser, um diese Seite korrekt darstellen zu können.

Vielen Dank für Ihr Verständnis.