232
Dioptrics of the Eye
[199, 200.
contained in the retinal image of the pupil of the observer’s eye, no
matter whether this image is clear-cut or blurred.
Notes: 1. This proposition is true not only when the light proceeds
directly from the source to the illuminated eye and thence into the observer’s
eye, but also when any lenses or mirrors are interposed along the way. Inci¬
dentally, this fact affords a convenient way of showing experimentally how an
ophthalmoscope acts in one’s own eye. The light used for illumination is
adjusted and the instrument arranged in the same position in front of the
observer’s eye as it would.be in front of the patient’s eye; then the part of the
field that is bright corresponds to the part of the retina that is illuminated.
It is possible to tell whether this bright field is large or small, and whether it is
uniformly illuminated or whether there are dark places in it, and how dark
they are. Then the source of light is transferred behind the instrument where
the observer’s eye would naturally be, so that the light shines through the
peephole. Whatever is illuminated now in the field of view is comprised in the
part of the retina that the observer will be able to see. This is the simplest and
easiest way of getting a clear idea of the effects of various combinations of
flat and curved mirrors and of convex and concave lenses in ophthalmoscopes;
without having to make complicated geometrical constructions that are oft-
times more confusing than helpful to the unintiated.
2. The effect of the mode of illumination described in this section is easily
regulated by the above rule. Everyday experience teaches us (as can be
proved also by a simple construction of the procedure of the light) that the
blurred image of a distant object cannot cover the sharp image of a nearer
object that is seen distinctly, but that the blurred image of a near object may
cover the sharp image of a more distant one. In the experiment with the
perforated mirror the blurred image of the peephole which must be adjusted
as near as possible in front of the patient’s eye overlaps the image of the more
distant source of light which is perhaps sharply in focus. If no mirror is
employed, so that the observer looks past the light into the patient’s eye, the
flame and the observer’s eye seem to the patient to be near together, and
when his eye is not nicely accommodated for them, their blurred images are
fused together. With illumination by a transparent plate of glass, both the
image of the light and that of the pupil of the observer’s eye may be sharply
in focus. The former is seen reflected in the plate and the latter is seen through
the plate, so that they come together on each other. Accordingly, it is best
for the patient himself to adjust the glass plate so that his eye looks luminous
to the observer. All that he has to do is to be careful that the observer’s eye
appears to be covered by the reflected image of the source of light.
A reciprocity law similar to that given above for light proceeding
in opposite ways over the same path may also be formulated for the
amount of light transmitted to and fro. In this connection, let us
state here, first, the following
Fundamental Law of Photometry.
Suppose a and h are the areas of two tiny elements of surface in a
transparent medium at the distance r apart; and let the brightness of
the luminous element at a be denoted by H ; then the quantity of
light received by h will be
H.ab cos a cos ß