464 
NATURE 
[June 24, 1915 

closed, look with the other eye past the pencil at some 
distant object; a narrow penumbra will be seen round 
the tip of the pencil, and on observing this carefully 
it will become evident that there are really two over- 
lapping images of the pencil-tip standing side by side, 
the portion common to the two being dark (Fig. 2, B). 
The nearer the eye is to the pencil, the greater is the 
separation of the images; in daylight, separation is 
just visible (to me) at a distance of about 3 ft. If the 
right half of the pupil is now covered by a card, the 
left image disappears; on covering only the left half of 
the pupil, the right image disappears. , If the pencil is 
placed in a horizontal position, the appearance is quite 
different; the pencil now appears sharply defined 
laterally, but its tip ends in a penumbra (Fig. 2, C). 
A > 
Fic. 2. 
It appears to me that these phenomena may be 
ascribed to the peculiar shape of the cornea. It has 
long been known that the cornea is not spherical, and 
Sulzer has found that its form does not agree with any 
known simple surface, and that it has no axis of 
symmetry. In the majority of cases the nasal side of 
the cornea is flatter than the temporal side, so that 
the section of the cornea of the right eye, 
when viewed from above, resembles BC, Fig. 3. 
The visual line OA (i.e., the line along which 
the most direct ray travels from the object 
O to the most sensitive portion of the retina A) 
passes through the flatter portion of the cornea; and 
the centre of the pupil is also behind the flatter portion 
of the cornea. Thus when the 
light is good, and therefore 
\ the pupil is small, the rays which 
form the image on the retina 
: pass through the flatter portion 
\ of the cornea; and under these 
conditions we obtain the best 
ocular images. 
Now, in aiming with the 
rifle in a dim light, the bull’s- 
eye being focussed, if the cornea 
were spherical, there would be 
a number of overlapping 
images of the fore-sight, thus 
giving rise to the appearance 
of a single dark image sur- 
rounded by a penumbra. The 
peculiar shape of the cornea, 
oO \ 
a 
however, appears to cause a 
segregation of these images 
into two. groups, giving 
rise to two overlapping images side by side. 
The light which enters the right eye through 
the left part of the cornea (i.e., the flatter 
portion) gives rise to the right-hand image; that which 
enters through the right (more strongly curved) por- 
tion of the cornea gives rise to the left image. So far 
as my experience goes, the right image is the darker 
and better defined of the two; and we might expect 
this to be the case, since it is formed by the rays which 
traverse that part of the cornea which is utilised when 
vision is at its best. It therefore appears that the 
right-hand image of the fore-sight should be aligned 
with the middle of the notch of the rear sight, its tip 
being just below the bull’s-eye at ‘* six-o’clock,”’ and 
NO. 2382, VOL. 95] 

+ just level with the top of the penumbra that bounds the 
shoulders of the V or U rear-sight (Fig. 4). In a dim 
light it is well to allow for the fact that the bull’s- 
eye is apparently raised, by leaving a distinct white 
line between the tip of the fore-sight and the lower 
side of the bull’s-eye. In all cases the fore-sight 
should at first be aligned some distance below the 
bull’s-eye, and raised to its final position just before 
firing. 
When the rifle is aimed in daylight with a bright 
sky overhead, light is reflected from the upper rim of 
the rear-sight into the eye. When the bull’s-eye is 
focussed, this light forms three bright linear images 
in the eye. The lowest bright line occupies the posi- 
tion of the upper boundary of the black portion of 
Fig. 4; the middle bright line occupies the position 
of the upper boundary of the penumbra shown in 
Fig. 4; while the upper bright line bounds a faint 
secondary penumbra which is scarcely visible in a dim 
light. Similarly, if a diaphragm with a narrow hori- 
zontal slit is placed in front of an eye focussed to see 
distant objects, three bright images of the slit are 
seen. These multiple images, which vary somewhat 
in position for different observers, and even for the 
two eyes of a single observer, are presumably due to 
variations of curvature of the cornea in a vertical 
plane. Correct shooting can be obtained by aligning 
the top of the fore-sight with the central bright line 
which bounds the lower penumbra; as this line is 
clearly seen, it can be utilised as easily as the focussed 
image of the rear-sight. The advantage of a good 
overhead light thus becomes apparent. 
So far as the lighting of indoor ranges is concerned, 

Fic. 4. 
it may be inferred that we shall see best under those 
conditions which approximate most closely to ordinary 
diffused daylight. The use of a small brightly illumin- 
ated target, in a room with black walls and ceiling, 
could be defended only if it were desired to train 
people to shoot at a distant searchlight. In such condi- 
tions the pupil is distended, all of the troubles discussed 
above are intensified, with the addition that the glare 
of the target tires the eyes. Similarly, the glowing 
filament of an incandescent lamp tires the eye more 
when it is viewed in a dark room than when it is 
viewed in daylight. I believe that the best thing to do 
in connection with indoor rifle ranges would be to 
whitewash the walls and ceiling, and have a good 
illumination either with electric glow lamps or incan- 
descent gas mantles, merely taking the precaution 
that the lamps are shielded (say, by paper shades) from 
the direct view of the shooters. 
So far as the utility of miniature rifle ranges is 
concerned, it appears to me that this may be easily 
overrated. It is possible, of course, at one of these 
ranges to learn to hold the rifle correctly, to become 
accustomed to accurate sighting, and to press the 
trigger without moving the rifle. Difficulty, however, 
arises from the fact that accurate shooting entails 
compliance with all three of these requirements, and 
bad shooting may be due to a failure in one only. 
The position of the bullet-hole in the target gives only 
the net result of all the actions involved; and I have 
known men to ascribe their failure to get near the 
bull’s-eye to the defective sights of the rifle, or 
- (more rarely) to their own defective sighting, when in 
