Physiology: The Science of the BodyMartin, Ernest G.
Science
Physiology: The Science of the Body
Martin, Ernest G.
Physiology
find out just how much the capsule must be loosened to give the proper
adjustment for any distance. As people get along in years the
crystalline lens often loses its elasticity so that it does not bulge
when the capsule is loosened. After this happens vision for near objects
is no longer clear. Relief is obtained by wearing reading glasses. These
are merely glass lenses worn in front of the eyes selected so that their
focus fits in with that of the lenses of the eye itself to bring the
image of objects at reading distance sharply on the retina. It is
usually necessary to replace these glasses from time to time as the
crystalline lens becomes stiffer and stiffer and ordinary accommodation
fails more and more.
There is a disease known as _cataract_ in which the crystalline lenses
become cloudy and finally completely opaque. Of course this means
blindness, since the passage of light to the retina is interfered with.
Relief is obtained by the simple expedient of removing the opaque lenses
bodily. This is possible merely because the crystalline lens is not the
chief lens of the eye. To be sure the cornea by itself will not focus on
the retina, but a glass lens can be placed in front of it which will add
itself to the cornea and the combined lenses will. There is no
possibility of accommodation in a case like this, so the patient has to
be furnished with bifocal lenses; the main part gives clear distance
vision; the lower section gives clear vision at the reading distance.
The patient has to get along with blurred vision in the regions between.
Not all eyes are exactly the right size so that distant objects shall
focus sharply on the retina. In fact a large proportion of them are
either too long or too short. It is clear that in an eyeball that is too
long the image of distant objects will fall in front of the retina, but
near objects that happen to be at just the right distance will focus
exactly on the retina. The distance at which this happens depends, of
course, on how much too long the eyeball is. Persons that have unduly
long eyeballs are, therefore, nearsighted. The condition is called
_myopia_. The correction for it consists in the use of lenses in front
of the eyes that instead of shortening the focus shall lengthen it.
Concave lenses, namely, those that are thick at the edge and thin in the
middle, will do this, and these are the kind that are worn by
near-sighted people.
When the eyeball is too short the image of distant objects falls behind
the retina, and of course that of near objects tends to fall farther
back yet. Since by
[Illustration: Photo, Fifth Avenue Hospital
SOFT, RESTFUL COLORS AND ATTRACTIVE SURROUNDINGS ARE CONDUCIVE TO GOOD
SPIRITS AND THEREFORE TO HEALTH]
[Illustration: Copyright, Paul Thompson
THE MAGNET IS HERE PUT TO THE UNUSUAL SERVICE OF REMOVING IRON FILINGS
FROM THE EYE]
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