Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
214, 215.] 
§16. Illumination of the Eye and the Ophthalmoscope 
When normal eyes are examined without the use of atropin, the 
same brightness might be obtained by means of both methods of 
illumination, provided the pupils of the two eyes did not alter their 
sizes. But the silvered mirror reflects on the whole more light into the 
illuminated eye and causes the pupil to contract more; so that under 
these circumstances the transparent mirror may give a larger field of 
view and a greater brightness. Besides, it illuminates the visible 
surface of the retina uniformly, whereas the blurred image of the hole 
in the other type of mirror has the effect of a shadow, so that the 
illumination is greater at some places than at others. And, lastly, in 
case of the unsilvered mirror the corneal reflex is less troublesome, 
because the light reflected from the mirror is more or less polarised, 
and on being reflected from the cornea without change of polarisation, 
most of it fails to get through the transparent glass plate or plates. 
In order that the unsilvered mirror shall reflect as much as half of 
the incident light, it may consist of a single glass plate or of several such 
plates put together, only the mirror must be inclined at a suitable 
angle depending on the number of plates. For one plate the proper 
angle of incidence is 70°, for three plates it is 60°, and for four plates it 
is 56°. 
Various types of ophthalmoscopes 
1. Helmholtz’s ophthalmoscope with reflecting glass plates and concave 
lenses; shown in section and actual size in Fig. 106; and as seen from in front, 
half-size, in Fig. 107. The illustrations include an improvement in the original 
construction, which was added by the instrument-maker Rekoss and which 
consists in two rotatable discs containing the requisite concave lenses. The 
three glass plates constituting the 
mirror are designated by aa. These 
form the sloping face of a rectangular 
prism box, the bottom of which is a 
right triangle, as seen in section in 
Fig. 106. The two perpendicular 
sides of this hollow prism are made of 
metal plates, covered on the inside 
with black velvet to absorb the light 
as much as possible. The smaller one 
of the metal plates is fastened to the 
frame of the instrument in such fashion 
that it can be rotated around the opti¬ 
cal axis; and there is an opening in it 
corresponding to this axis. The glass 
plates are held against the prismatic 
box by a rectangular frame; and the 
frame itself is fastened to the triangular 
base of the prism by two screws ee. The glass plates are inclined to the optica 
axis of the instrument at an angle of 56°. 
Moreover, two discs bh and cc turn around an axis dd inserted in the metal 
frame gg; and each of these discs has five circular openings, four of which 
Fig. 106.


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