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.
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.
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.
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.