Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
VISION. 1443 
'contraction of the pupil, arresting the progress 
of those rays which are too divergent, and 
limiting those entering the eye to the central 
rays, which, from their trifling divergence, 
unite correctly on the retina. The same ad¬ 
vantage is gained by a near-sighted, and by a 
presbyopic eye. In the one case vision is im¬ 
proved by the card stopping the rays, which 
would converge to foci in front of the retina ; 
in the other, by its arresting those which tend 
to foci behind the retina. So that this simple 
experiment frequently makes a difference of 
several inches in the vision of myopic and 
presbyopic persons. 
A curious experiment devised by Father 
Sch einer, has reference to this point. If we 
make in a card two small orifices with a 
needle, at a less distance from each other 
than the diameter of the pupil, and hold 
these openings close to the eye, a double 
image of a small object held within the 
visual distance, — a pin’s head, for instance— 
will be seen. From the pin’s head there pass 
two very minute pencils of rays through the 
apertures into the eye. These rays converge 
towards a point lying behind the retina, and 
fall upon the retina at two different points. 
These are two isolated points of the circle of 
dispersion, which would exist upon the retina, 
if the other rays were not intercepted by the 
card. If we now withdraw the pin’s head more 
and more, the images will approach, because 
the rays, falling upon the eve through the 
apertures, will diverge less, and will conse¬ 
quently be refracted towards a point lying 
nearer to the retina. If the object be re¬ 
moved from the eye to the distance of distinct 
vision, the two images will perfectly coincide, 
since all rays passing from one point, lying 
exactly at the distance of distinct vision, will 
be concentrated at one point of the retina. It 
may be asked, what are the conditions of 
adaptation necessary for an eye in looking 
through a fine aperture? In its normal con¬ 
dition, for the maintenance of which no effort 
is necessary, the eye is in the state necessary 
for seeing objects which lie at the distance of 
distinct vision. If a distant object be re¬ 
garded through the small apertures, the rays 
passing through them into the eye must evi¬ 
dently meet at one point before the retina, as 
the condition of each adaptation does not 
change in the eye: but the two pencils di¬ 
verge again behind the point of intersection, 
striking the retina at two different points, 
when, consequently, distant objects will be 
seen double ; therefore, we only see a small 
object single, through two small apertures, 
when it lies at the distance of distinct vision. 
This experiment of Scheiner led Dr. Porter¬ 
field to invent an instrument called an op¬ 
tometer, for the determination of the focal 
distance of the eye; and Dr. Young sub¬ 
sequently greatly improved upon it, his in¬ 
strument being simple in construction, and 
both convenient and accurate in its applica- 
The greatest distance of human vision is 
so variable, that no arbitrary limits can be as¬ 
signed to it. Uncivilised tribes, as the North 
American Indians, the inhabitants of the im¬ 
mense Asiatic steppes, and the New Zealan¬ 
ders, possess powers of sight which are 
almost incredible. It is interesting, however, 
to remark, that the mean degree of capability 
of vision was the same among the ancient 
Greeks and Romans as at the present day. 
The Pleiades furnish the proof of this, showing 
that some thousand years ago, as now, stars 
which astronomers call of the seventh mag¬ 
nitude, were not visible to the naked eye in 
persons of ordinary powers of vision. Even 
among civilised nations, however, instances 
are occasionally met with of extraordinarily 
keen sight. General Drinkwater, in his 
*■ History of the Siege of Gibraltar,” mentions 
that there were two boys in the garrison pos¬ 
sessed of such uncommon quickness of sight, 
that they could see the shots fired by the 
enemy almost immediately after they had 
quitted the guns, and were constantly em¬ 
ployed to look out and give warning to the 
soldiers of the approach of these missiles. 
From the experiments of Harris, it seems 
that a simple object, as a black square on a 
white ground, or a white square on a black 
ground, can be seen under a less angle than 
the equal parts of a compound object, as the 
squares of a chequered figure, and that their 
least, or minimum visible angle cannot be less 
than 40" : others, however, say 30". If it is 
40", the size of the image on the retina will 
be -söVö inch. At a medium, Harris thinks it 
not less than 2'. He remarks that the diffi¬ 
culty of keeping the eye perfectly steady, may 
be one cause why a single object can be dis¬ 
cerned under a less angle than the parts of a 
complex one ; and that it is natural to sup¬ 
pose that the fewer objects we contemplate, 
and the more they differ in colour, the easier 
we can distinguish their several impressions on 
the retina. The result of repeated and very 
careful experiments by Hueck, tends to show' 
that white objects on a black ground are seen 
at a greater distance than black objects on a 
white ground *, and this is fully corroborated 
by an instance mentioned by Humboldt. This 
distinguished traveller was with Bonpland, at 
* The following facts, deduced from extensive 
and very careful experiments, conducted by Lieut.- 
Colonel Hamilton Smith, are of great practical im¬ 
portance. The object was to ascertain the liability 
of different colours to be hit as marks, under pre¬ 
cisely similar circumstances, as to men engaged, size 
of target, weather, &c. The result showed the 
proportion to be as follows. — 
Red, 12. Rifle-green, 7. Austrian blueish-grey, 5. 
Colonel Derinzy, who was actively engaged in 
the Peninsular War, has given much attention to 
the effect of different coloured uniforms on the 
chances of being hit. The day before the battle of 
Yittoria, his Portuguese rifle company, dressed in 
earthy-brown, and a company of British Fusil eers, 
of equal strength, dressed in red, had to dislodge the 
French from a bridge. They were equally exposed 
during the whole of the skirmish, and after it was 
over, Colonel Derinzy compared notes with his 
Fusileer comrade, and found that the relative losses 
were precisely one to two ! This fact (for which we 
are indebted to Captain Nelson, k.e.) strikingly 
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