does not escape during this step of the operation, and neither the lens nor the iris can be wounded. Wounds of the iris, however, although well to be avoided, are seldom productive of any inconvenience.

As the anterior chamber is emptied, the iris gradually advances until it comes into complete contact with the cornea, and the tension of the globe diminishes in a degree proportionate to the quantity of the aqueous humour. If the state of the anterior chamber be normal, the pupil dilates a little, yielding to the forward pressure of the lens. After a few minutes the chamber is refilled as before; although the rapidity with which the aqueous humor is restored varies much in the various diseases of the eye. In order to empty the chamber again, the wound is re-opened by a little probe, the simplest introduction of which is often sufficient. Sometimes it is necessary to use a little backward pressure, since the iris, closely applied to the cornea at the puncture, may impede the escape of the fluid. Such pressure is the only part of the procedure that may be a little painful, by its influence upon the ciliary circle and plexus of nerves. The sensitiveness of these parts is a reason for not losing sight of the exact place of puncture, and for not making painful exploration with the probe for its discovery. An increase of sensitiveness in the eye is never to be considered a contra-indication for paracentesis; since such sensitiveness and the accompanying tension and pain, are nearly always removed by successive evacuations.

The forward pressure of the iris and lens is the immediate cause of the escape of the aqueoue humor. When there is posterior synechia, this pressure is not exercised, and the fluid no longer escapes spontaneously. It is then necessary to introduce the probe repeatedly, and to introduce a stronger pressure, in order to cause the escape of the humour. During the first days of the treatment, it is often impossible to empty the anterior chamber completely.

When the chamber contains concrete pus, or lymph, it is sometimes necessary to carry the probe across the morbid product, in order to cause the escape of the humour; and, under similar circumstances, it may be needful to introduce the probe several times, especially if care has not been taken to make an opening sufficiently large. In such cases the blunt whalebone probe should be preferred, since it is less liable to bruise, or injure the iris.

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The evacuations of the aqueous humour by the same apperature may be effected by two methods-either repeatedly in a single seance, followed by an interval of some days, or singly, at shorter intervals, and for a considerable time.

By the first method the anterior chamber is emptied, at the same seance and by the same opening, twice, thrice, four or even morė times, at intervals of some minutes, according to the quantity of the fluid and the rapidity of its reproduction. By the second method, the probe is introduced and the chamber emptied in the morning, and again in the evening, or several times a day, and this for many days, or even weeks. Sometimes it may be necessary to combine these methods, and sometimes it will suffice to empty the chamber once every day.

Commonly, after thirty or forty hours, the corneal opening is found closed, and it is necessary to use a little pressure with the probe, in order to break down the adhesions. Excepting for the trifling pain, the wound may be re-opened after an interval of eight or ten days, without the least inconvenience.-Ophthalmic Review, London.

AN INTRODUCTION TO THE STUDY OF THE OPTICAL DEFECTS OF THE EYE AND THEIR TREATMENT BY THE SCIENTIFIC USE OF SPECTACLES.

(Continued from No. 7, page 270.)

BY A. M. ROsebrugh, m. D., TORONTO, c. w.

CHAPTER III-MYOPIA.

Concave Lenses.-Before proceeding to the consideration of myopia, it will be well for us to glance at some of the properties of concave lenses; and in order to simplify the subject, we will confine

ourselves to equi-concave lenses. An equi-concave lens is bounded by two surfaces, which are portions of the concave side of two circles which have equal radii.

Fig. 9. A, B, one of the concave surfaces of the lens. C is the centre of curvature, and C, R the radius of curvature. When parallel rays, P, P, strike one surface of the lens, they have a divergence upon leaving the second surface of the lens, as if they proceeded from the centre of curvature, C, which, in an equi-concave lens, is also the principal focus of the lens. C, R, is the focal length of the lens. In a convex lens, the focus is measured behind the lens; in a concave lens, it is measured in front of it. If we call the focus of the convex lens positive, we must call the focus of the concave lens negative. When parallel rays of light fall upon a convex lens, they are converged to a focus. When they fall upon a concave lens, they are made to diverge. A convex lens enlarges, and a concave diminishes the apparent size of objects. The focal length of a convex lens is measured behind; and that of a concave lens, in front of the lens. They are, therefore, entirely opposite in all their properties; and, for this reason, a convex lens is called a positive lens; and a concave one, a negative lens. Or, shorter still, they are indicated by the plus (+) and minus (-), algebraic symbols; thus, + 5, and 5; or, +, and . To ascertain the focal length of a concave lens, we ascertain what convex lens it will neutralize.

1. In a myopic eye, parallel rays, as well as those that have a certain degree of divergence, are focussed in front of the retina; and, the inverted image of distant objects being formed in the same position, the picture upon the retina will be ill-defined, and vision for distant object consequently indistinct.

Patients with myopia complain that, although their vision for near objects is perfect, they cannot see objects at a distance with any dis

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tinctness. They can read the smallest type, when brought near the eyes, even better than persons with normal vision, but they are not able to recognize their friends at a distance of fifteen or twenty feet.

In order to enable such persons to see distinctly at a distance, it is necessary for them to wear concave spectacles of such a strength, that the parallel rays from distant objects may have such a degree of divergence, that, falling upon the myopic eye, they may form a focus upon the retina. Theoretically, we should prescribe concave glasses of such a strength that their focus will correspond with the patient's "far" point. Thus, if the "far" point be at 12 inches, we should prescribe 12, as a twelve inch concave lens, placed before such an eye, will give parallel rays from distant objects the same degree of divergence as if they proceeded from the "far" point of the eye; namely, at 12 inches from the eye. Thus, in Fig. 9, P. P. represent parallel rays falling upon the concave lens, A. B.; they are made to diverge, as if coming from the focus, C., and falling upon the eye divergingly, they are focussed upon the retina at F. Practically, however, we would find that 12 would be rather too strong, and that 16 would probably answer better. As a rule, the weakest glasses should be worn that will enable the patient to see distant objects with distinctness.

15, or

In testing the degree of myopia, we use a series of test types that are so constructed that No. I (smallest) can be distinctly seen and read by a person having normal vision, at a distance of 1 foot; No. II, at 2 feet; No. V, at 5 feet; No. XX, at 20 feet; and so on. A specimen of these types will be annexed to this paper. The types are also used in testing the acuteness of vision in Presbyopia, Hypermetropia, Amblyopia, &c.

2. In determining the degree of myopia in any case, we ascertain the greatest distance at which No. I test types can be read distinctly; if at 10 inches, the "far" point will be at 10 inches, and the myopia would be called; if at 6 inches, the myopia would be called . From this we can, as stated above, get a proximate knowledge of the strength of the concave lens necessary to relieve the myopia.

3. A myopic eye, when in a state of rest, is adjusted for diverging rays. To enable such an eye to see distant objects, that is, to bring parallel rays to a focus on the retina, it is necessary to give these parallel rays a preliminary degree of divergence by the interposition of the proper concave lens.

distinctly, No. I

We will suppose only read No. II

4. Myopia can be distinguished from every other defect of vision, by the fact that concave glasses improve vision for distant objects. If we have no concave glasses convenient, we can diagnose it from Amphyopia, (insensibility of the retina) by the following ready method:-A person with normal vision can read test type at 12 inches, and even a little farther. that a patient's vision is so impaired, that he can at 6 inches; if he is not also myopic, he can also read No. IV at 12 inches, or No. LX at 180 inches-that is at 15 feet. However impaired then a person's vision may be, unless he be also myopic, he can see as well proportionately, at one distance as at another. On the contrary, a person with myopia, say, can see the smallest type (much smaller than No. I,) at 6 inches, but he cannot see No. II, or even No. V, at 12 inches.

This disease is often hereditary. Over exertion of the eyes upon near objects at the age of puberty, (about 14 or 15) is a very frequent cause of myopia.

Short-sighted persons often inquire if we would advise the use of spectacles. There can be no objection to wearing glasses that will enable them to see distant objects; for their eyes are thus changed to normal ones, but as most persons use their eyes much more frequently upon near than upon distant objects; the glasses. should be no stronger than necessary. Some contend, however, that short-sighted persons should dispense with glasses for reading, writing, &c. Prof. Donders, however, recommends their use for this purpose, for the following reasons:—

1st. "Because strong convergence of the optic axes is necessarily paired with tension of the accommodation. The latter is an associated action, not arising from the mechanism of the convergence, but existing within the eve itself, and may consequently easily lead to an increase of the myopia. Besides this, the pressure of the muscles upon the eye ball appears to be greater when the optic axes are convergent, than when they are parallel, and this increase of pressure cannot but tend to give rise to the development of posterior staphyloma.

2d. "On account of tho habit which short-sighted persons have of bending their head forwards during reading or writing. This must cause an increased flow of blood to the eye, and an increased tension within the eye itself. Owing to this development of sclerotico-choroiditis posterior, effusions of blood and detachment of the retina