Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/1478/
14-68 
VISION. 
vision, a point appearing a line, a circle an 
oval, and a square a parallelogram. In an in¬ 
teresting case related by Dr. Robert Hamil¬ 
ton*, the patient, when looking at a clock, was 
unable to distinguish the hands if they pointed 
perpendicularly, as at six o’clock, but if hori¬ 
zontally he had no difficulty : so when looking 
at a wheel at a little distance, the horizontal 
spokes only could be seen. The patient was 
a coach painter by trade, and this peculiarity 
of vision greatly interfered with his business, 
for he could not draw vertical lines with any 
degree of correctness, and unwittingly made 
them slanting, a serious fault in heraldic de¬ 
vices ; horizontal lines he drew with perfect 
precision. His method of correcting the per¬ 
ceptions of perpendicular lines was to bend 
his head at right angles with his body, where¬ 
upon upright bodies became distinct and accu¬ 
rately represented. This man also practised 
a manoeuvre which forcibly reminds us of an 
act common to persons having conical corneae, 
that of placing the fore-finger at the outer 
angle of the eyelids and drawing them out¬ 
wards whereby vision is improved. 
To remedy the defect under which he 
laboured, Professor Airey made a pin-hole in 
a blackened card, which he caused to slide on 
a graduated scale ; then strongly illuminating 
a sheet of paper and holding the card between 
it and the eye, he had a lucid point on which 
he could make observations with ease and 
exactness. Then resting the end of the scale 
on the cheek bone he found that the point at 
the distance of 6 inches appeared a very well- 
defined line inclined to the vertical about 35° 
and subtending an angle of 2°. Again, at the 
distance of 3^ inches, it appeared a well-defined 
line at right, angles with the former, and of the 
same apparent length. It was therefore neces¬ 
sary to make a lens which, when the parallel 
rays were incident, should cause them to 
diverge in one plane from the distance of 3i 
and in the other plane from the distance of 
6 inches. The professor obtained a lens of 
which the radius of the spherical surface was 
3£ inches, of the cylindrical 4£ inches, and with 
this he was able to read the smallest print. 
In Dr. Hamilton’s patient the relation of the 
horizontal to the vertical focus appeared to be 
as inches to 6| inches, and on trying him 
with- plano-concave cylindrical lenses, it was 
found that a lens of 24 inches focal length, 
the cylindrical surface being made to act ho¬ 
rizontally, operated very beneficially. Besides 
this irregular refraction the man was myopic, 
but the lenses in question fitted as spectacles, 
enabled him to see weil. 
The defect of the cylindrical eye may be de¬ 
tected by making a small pinhole in a card 
which is to be moved from close to the eye to 
arm’s length, the eye meanwhile being directed 
to the sky, or any bright object of sufficient 
size. With ordinary eyes the indistinct image of 
the hole remains circular at all distances, but 
to an eye having this peculiar defect it be¬ 
comes elongated, and when the card is at a 
* Monthly Journal of Medical Science, June 1847. 
certain distance passes into a straight line. 
On further removing the card the image be¬ 
comes elongated in the perpendicular direc¬ 
tion, and finally if the eye be not too 
long-sighted, passes into a straight line per¬ 
pendicular to the former. 
Professor Stokes has invented a highly 
ingenious instrument for determining the 
nature of the required lens, and the following 
is the proposition on which it is based. 
Conceive a lens ground with two cylindrical 
surfaces of equal radius, one concave and the 
other convex, with their axes crossed at right 
angles ; call such a lens an astigmatic lens : 
let the reciprocal of its focal length in one of 
the principal planes be called its power, and a 
line parallel to the axis of the convex surface 
its astigmatic axis. Then if two thin astig¬ 
matic lenses be combined with their axes 
inclined at any angle, they will be equivalent 
to a third astigmatic lens determined by the 
following construction. 
From any point draw two straight lines 
representing in magnitude the powers of the 
respective lenses, and inclined to a fixed line 
drawn arbitrarily in a direction perpendicular 
to the axis of vision at angles equal to twice 
the inclinations of their astigmatic axes, and 
complete the parallelogram. Then the two 
lenses will be equivalent to a single astigmatic 
lens represented by the diagonal of the pa¬ 
rallelogram in the same way in which the 
single lenses are represented by the sides. 
A piano-cylindrical or spherico-cylindrical 
lens is equivalent to a common lens, the 
power of which is equal to the semi-sum of 
the reciprocals of the focal lengths in the two 
principal planes, combined with an astigmatic 
lens, the power of which is equal to their 
semi - difference. If two piano-cylindrical 
lenses of equal radius, one concave and the 
other convex, be fixed one in the lid and the 
other in the body of a small round wooden 
box, with a hole in the top and bottom, so as 
to be as nearly as possible in contact, the 
lenses will neutralise each other when the 
axes of the surfaces are parallel ; and by 
merely turning the lid round, an astigmatic 
lens may be formed of a power varying con¬ 
tinuously from zero to twice the astigmatic 
power of either lens. When a person who 
has the defect in question has turned the lid 
till the power suits his eye, an extremely 
simple numerical calculation, the data of 
which are furnished by the chord of double 
the angle through which the lid has been 
turned, enables him to calculate the curva¬ 
ture of the cylindrical surface of a lens for 
a pair of spectacles which will correct the 
defect in his eye.* 
A curious case is related in the Annales 
d’Oculistiquef, of an anomaly of vision, 
which was probably the consequence of a 
defect in the form of the cornea, such as 
that under consideration. M. Schnyder, the 
Pastor of Menzberg in the Canton of Lucerne, 
* Eeport of the British Association, vol. xviii, 
f Tom. xxi. p. 222.
        

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