Dr. Javal has just presented to the Academy of Medicine a very
ingenious and practical optometer devised by George J. Bull, a young
American doctor, after a number of researches made at the laboratory
of ophthalmology at the Sorbonne. Among other applications that can be
made of it, there is one that is quite original and that will insure
it some success in the world. It permits, in fact, of approximately
deducing the age of a person from certain data that it furnishes as to
his or her sight. As well known, the organs become weak with age,
their functions are accomplished with less regularity and precision,
and, according to the expression of the poet,
"_En marchant a la mort, on meurt a chaque pas,_"
the senses become blunted, the hearing becomes dull, the eyes lose
their luster, vivacity, and strength, and vision becomes in general
shorter, less piercing, and less powerful.
The various parts of the eye, but more particularly the crystalline
lens, undergo modifications in form and structure. Accommodation is
effected with more and more difficulty, and, toward the age of sixty,
it can hardly be effected at all.
These changes occur in emmetropics as well as in hypermetropics and
myopics.
As will be seen, then, there is a relation between the age of a
person and the amplitude of the accommodation of his eyes. If we
cannot express a law, we can at least, through statistics, find out,
approximately, the age of a person if we know the extent of the
accommodation of his eyes.
A Dutch oculist, Donders, has got up a table in which, opposite the
amplitudes, the corresponding ages are found. Now, the Javal-Bull
optometer permits of a quick determination of the value of the
amplitude of accommodation in _dioptries_. (A dioptrie is the power of
a lens whose focal distance is one meter.)
The first idea of this apparatus is due to the illustrious physicist
Thomas Young, who flourished about a century ago. The Young apparatus
is now a scarcely known scientific curiosity that Messrs. Javal and
Bull have resuscitated and transformed and completed.
It consists of a light wooden rule about 24 inches long by 1¼ inch
wide that can easily be held in the hand by means of a handle fixed at
right angles with the flat part (Fig. 1). At one extremity there is a
square thin piece of metal of the width of the rule, and at right
angles with the latter, but on the side opposite the handle. This
piece of metal contains a circular aperture a few hundredths of an
inch in diameter (Fig. 3). Toward this aperture there may be moved
either a converging lens of five dioptries or a diverging lens of the
same diameter, but of six dioptries.
[Illustration: FIG 1.--MODE OF USING THE BULL OPTOMETER]
On holding the apparatus by the handle and putting the eye to the
aperture, provided or not with a lens, we see a series of dominoes
extending along the rule, from the double ace, which occupies the
extremity most distant from the eye, to the double six, which is very
near the eye (Fig. 2). The numbers from two to twelve, simply, are
indicated, but this original means of representing them has been
chosen in order to call attention to them better.
[Illustration: FIG 2.--THE RULE, WITH THE DOMINOES (¼ Actual
Size.) ]
Figures are characters without physiognomy, if we may so express
ourselves, while the spots on the dominoes take particular
arrangements according to the number represented, and differentiate
themselves more clearly from each other than figures do. They are at
the same time more easily read than figures or regularly spaced dots.
Now, it is very important to fix the attention upon the numbers, since
they are arranged at distances expressed in dioptries and indicated by
the number of the spots. On looking through the aperture, we see in
the first place one of the dominoes more distinctly than the rest.
Then, on endeavoring to see those that are nearer or farther off, we
succeed in accommodating the eye and in seeing the numbers that
express the extreme terms of the accommodation, and consequently the
amplitude.
[Illustration: FIG. 3.--DETAILS OF EYE PIECE.]
Let us now take some examples: If we wish to express in dioptries the
myopia of a person, we put the apparatus in his hand, and ask him to
place his eye very near the aperture and note the number of spots on
the most distant domino that he sees distinctly. This is the number
sought. If the observation be made through the upper lens, it will be
necessary to subtract five from the number obtained; if, on the
contrary, the other lens is used, it will be necessary to add six.
If it is a question of a presbyope, let him look with his spectacles,
and note the nearest domino seen distinctly. This will be the number
of dioptries expressing the nearest point at which he can read. This
number permits us to know whether it is necessary to add or subtract
dioptries in order to allow him to read nearer by or farther off. If,
for example, he sees the deuce and the ace distinctly, say 3 dioptries
or 0.33 meter, and we want to allow him to read at 0.25 meter,
corresponding to four dioptries, it will be necessary to increase the
power of his spectacles by one dioptrie.
Upon the whole, Dr. Bull's optometer permits of measuring the
amplitude of accommodation, and, consequently, of obtaining the
approximate age of people, of knowing the extreme distances of the
accommodation, and of quickly finding the number of the glass
necessary for each one. It reveals the defects in the accommodation,
and serves for the quick determination of refraction. So, in saying
that this little instrument is very ingenious and very practical, Dr.
Javal has used no exaggeration.--_La Nature._
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