214, 215.]
§16. Illumination of the Eye and the Ophthalmoscope
249
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
JU
Fig. 106.