Volltext: Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition (1)

VI. Ophthalmoscopy 
The only disadvantage of this method of investigation with un¬ 
dilated pupil is the small field. In order to get rid of the corneal 
reflex when it is desired to obtain a larger field with a smaller pupil, 
an image of the source of light has to be formed near the narrowest 
part of the region of radiation of the catadioptric system, that is, near 
the image of the hole reflected in the cornea, or near the pupil of the 
observer. This is accomplished by combining a suitable aplanatic 
convex lens with a plane parallel plate of glass and using the Nernst 
slit lamp as source of light. If the distance of the lens from the plate 
of glass used as a mirror is correctly measured, an image of the slit 
may be produced in the pupillary plane of the patient’s eye. It is 
found also that the corneal reflex may be abolished without difficulty 
by small movements of this ophthalmoscope, and that the size of the 
illuminated field is quite large; but very little can be seen in this field 
due to the hindrance of foreign light. It is not hard to understand that 
the cause of this obscurity is light that is reflected diffusely in the cornea 
and in the crystalline lens. All that is necessary in order to see this is 
to separate the eye and mirror a little. This experiment shows that, 
when more is required with respect to the extent of the field of the 
illumination system, it is not enough merely to abolish the corneal 
reflex, but that the light diffusely reflected in the cornea and in the 
crystalline lens must be warded off also. 
However, provided we are content to have a smaller field, the 
following is a very useful method also: A lens of refracting power 
about 30 dptr and not more than 10 mm in diameter, combined with a 
plane mirror perforated by a hole 2 mm in diameter, makes a very 
suitable instrument for investigating the macula without dilating 
the pupil. With a horizontal slit a concave mirror may also be used, 
either by itself or in combination with a convex lens, depending on 
the curvature of the mirror. 
Moreover, in the method of the inverted image, the corneal reflex 
may be abolished without difficulty, provided a suitable aplanatic 
ophthalmoscope-lens is available, such as can be had nowadays. In 
order to construct the region of radiation of the first catadioptric system 
in the medium where the light travels before it is reflected at the cornea, 
the lens of the ophthalmoscope and the image reflected in the cornea 
must be considered as the apertures; because, after being reflected 
from the cornea of the patient’s eye, the light must pass through 
the lens of the ophthalmoscope before entering the pupil of the observ¬ 
er’s eye as a corneal reflex. The maximum extent of the radiation 
space includes all the straight lines that can be drawn through these 
two apertures. Now if the image of the pupil of the patient’s eye 
made by the lens of the ophthalmoscope falls outside this region of


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