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/lit29465/1462/
VISION. 
1452 
covers, and is inserted into, the anterior one- 
eighth of an inch of the choroid membrane, 
which is in this part tougher and firmer than 
elsewhere, and united in a very special manner 
to the lens by the ciliary processes, through 
the medium of a firm tough membrane, and 
of a strong elastic fibrous membrane proceed¬ 
ing from it to the margin of the lens, and yet 
not quite to its margin, for an elegant arrange¬ 
ment exists, the canal of Petit, by which 
traction is made, not on the vitreous around 
the lens, nor on the edge of the lens itself, so 
much as on its anterior surface. I confess it 
seems to me very difficult to doubt that this 
complicated system of parts is intended to 
advance the lens towards the cornea, so as to 
bring forward, up to the retina, the focus of 
a near object, which would otherwise fall be¬ 
hind the nervous sheet. It is possible also, I 
think, from the peculiar direction taken by the 
ciliary muscle, that it may compress the front 
of the vitreous, and thus help to throw for¬ 
ward the lens.” * 
An ingenious theory has been suggested by 
Sturm, and supported by Matteuci, founded 
on the results of Chossat’s measurement of 
the eye of an ox, to the effect that in place 
of comparing the optical apparatus of the eye 
to a system of spherical lenses whose axes are 
blended, we ought to consider the organ as 
composed of several refracting media, sepa¬ 
rated by surfaces which are neither exactly 
spherical, nor even of revolution or symme¬ 
trical about a common axis. Reasoning from 
this, he argues “that a peculiar refraction of 
the rays of light takes place, whereby tne 
retina is placed in what he terms a focal in¬ 
terval, which focal interval will change its 
position according as the external luminous 
point recedes from, or approaches to, the eye 
and that the retina will be always met by the 
concentrated fasciculus around the axis in the 
focal interval ; the surface of intersection of 
this fasciculus and of the retina being very 
slightly modified, in order that the impression 
may not be sensibly altered, or the perception 
rendered indistinct. This theory is, however, 
decidedly open to objection, and is rendered 
unnecessary by that of Mr. Bowman. 
Magnifying lens. — It has been already 
stated that the apparent magnitude of an ob¬ 
ject depends upon that of the angle of vision 
under which it is seen, and this increases in 
proportion as the object is brought nearer to 
the eye ; but the magnitude of the angle of 
vision being limited, we are obliged to resort 
to artificial means to enlarge it further than, 
in its natural condition, is admitted of. The 
pin-hole aperture affords some assistance, but 
the convex lens more. The following is the 
mode in which it acts : 
Let cd be a convex lens, and ab an object 
lying within the focal length of the glass, then 
all the rays passing from a point ot the object 
ab will diverge after their passage through 
the lens, exactly as if they came from the 
corresponding point of the image ab ; an eye 
* Lectures on the Parts concerned in the Opera¬ 
tions on the Eve, p. 60. 
Fig. 889. 
a 
behind the lens would be able to see the ob¬ 
ject distinctly through the lens if the image 
ab were at the distance of distinct vision. In 
this case, however, the object being much 
nearer the eye, it could not be seen without 
the lens. The magnifying power of the lens 
therefore depends essentially on the means it 
gives us of bringing the object very near the 
eye, and thus increasing the angle of vision. 
Abnormal Vision. 
In the consideration of abnormal vision, we 
propose to divide the subject into 
1. Abnormal Vision resulting from defec¬ 
tive action of the retina or sensorium ; as 
Achromatopsy, Hyper chromatopsy, and 
Anorthopia. 
2. Abnormal Vision arising from faulty con¬ 
figuration of the eye, or from changes in 
the refractive media ; as Myopia, Pres¬ 
byopia, and Cylindrical Eye. 
Amaurosis, Chrupsia, and other morbid 
conditions, do not fall within the scope of this 
article. 
Achromatopsy Ça not, xP®lJLa colour, Ünp the 
eye), or insensibility of the eye to colours, is 
an affection which has been recognised nearly 
two hundred years ; but, although cases have 
been from time to time published in the Phi¬ 
losophical Transactions and other scientific 
works, our knowledge of the phenomena of 
this singular condition is of recent date, and 
is chiefly due to the labours of Wartmann, 
Seebeck, Szokalski, Purkinje, Himly, &c. 
Various names have been proposed for this 
imperfection of vision ; but the majority are 
exceedingly unmanageable. By Sommer and 
Szokalski the term chromato-pseudopsis has 
been employed ; Goethe proposed to call it 
akyano-blepsis, whilst Purkinje divided the 
disorder into four varieties,—achromatopsia, 
chromât o-dy sop sis, akyano-blepsis, and anery- 
thro-blepsis ; others again have been satisfied 
with the simple term chromât o-metablepsiS. 
JUngken employs indifferently the denomina¬ 
tions of achromatopsy, chromatopseudopsy, and 
chromatomctablepsy. Many writers, however, 
have adopted the term Daltonism, proposed by 
Prévost, and supported by Wartmann ; and, 
although objectionable as perpetuating the in¬ 
firmity of an individual, it has the merit of 
simplicity and easy inflection. The term 
achromatopsy is, perhaps, that most usually 
employed, although, strictly speaking, it is 
only applicable to one class. Still, being ex¬ 
tensively recognised, we shall adopt it to de¬ 
signate this imperfection of vision ; occasion-
        

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