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the book a little nearer to her, than she had been previously accustomed to place it. The second case occurred in a patient about the same age, who, in the course of the last year, was attacked with an inflammation in both eyes. By the use of leaches and cooling medicines, it was speedily removed, and, afterwards, she was much gratified, by finding that the necessity for using glasses when she read, which had existed many years, was removed; and that she could see both near and distant objects correctly, without any extraneous help. The amendment in this lady's sight continued, however, only a few weeks; after which she was again obliged to use the same convex glasses in looking at small near objects, which she had used before her eyes became inflamed.-In addition to these cases, I beg leave to add the information I have re

I ceived from an eminent mathematical instrument maker, about fifty years

of

age, who has long made use of convex glasses · to assist his sight in reading. He tells me, that when he

has been employed many hours together, for several successive days, in looking through a double microscope that magnifies twenty-eight times, (in order to enable him to mark the degrees on a small brass plate) he has afterwards been able, repeatedly, for a few weeks, to read without his glasses; but then the amendment gradually ceases, and he is soon obliged to return to the use of the same glasses that he had worn before.

In the instances that have been mentioned, the distant sightedness affected persons who were considerably advanced in life: but in the three that follow, a similar affection of the sight occurred in those that were young; and a like good effect was produced by the use of evacuating remedies. One of these was a boy eight years old, who suddenly became presbyopic, and had repeatedly been punished at school, on account of his incorrect and defaced writing; the real cause of it, at that time, being unknown to his master. After the presbyopia had continued a fortnight, and different local applications had been used, without producing any sensibly good effects, the lad was cured by the application of leaches to the temples, and the administration of a few purgative medicines. The other instances occurred in two daughters of the same family. The eldest, twenty years of age, had never been able

, to do fine work, and for three years had been greatly assisted by convex spectacles. The youngest, a girl of fifteen, had become presbyopic about a year ago, and since that time had been obliged to use spectacles whenever she read, or worked with her needle. The young person, last mentioned, in the course of six weeks, (during which time she totally abstained from the use of glasses,) was completely relieved from the necessity of using them, by the application of two leaches to each temple twice in a week. The former, in the same space of time, experienced much relief from a similar treatment, but was still unable to do fine work without glasses, partly in consequence of the long continuance of the infirmity, and partly on account of her not having abstained with equal steadiness from the occasional use of them.

From the preceding statement, the following inferences may be deduced.

First; near sightedness is rarely observed in infants, or even in children under ten years of age. It affects the higher

. classes of society more than the lower: and the instances are few, if any, in which, if the use of concave glasses has been

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adopted, increasing years have either removed or lessened this imperfection.

Secondly; though the usual effect of time on perfect eyes be that of inducing a necessity to make use of concave glasses, in order to see near objects distinctly, yet sometimes, even after the age of fifty, and after convex glasses have been used many years for this purpose,

the

eyes have not only ceased to derive benefit. from them, when looking at near objects, but they have required concave glasses to enable them to distinguish, with precision, objects at a distance.

Thirdly; though the cause of this change be not always known, yet sometimes it has been induced by the use of evacuating remedies, particularly of leaches applied to the temples; and sometimes by looking through a microscope, for a continued length of time, in several successive days.

Fourthly; instances are not uncommon, in which persons, far advanced in life, (viz. between eighty and ninety,) whose eyes have been accustomed for a long time to the use of deeply convex glasses, when they have read or written, have ceased to derive benefit from these glasses, and they have become able, without any assistance, to see both near and distant objects almost as well as when they were young. Although it be not easy to ascertain the cause of this amended vision, it seems not improbable that it is occasioned by an absorption of part of the vitreous humour; in consequence of which the sides of the eye collapse, and its axis, from the cornea to the retina, is lengthened; by which alteration the length of this axis is brought into the same proportion to the flattened state of the cornea or crystalline, or both, which it had to these parts before the alteration took place.

V. The Bakerian Lecture. On the elementary Particles of certain

Crystals. By William Hyde Wollaston, M. D. Sec. R. S,

Read November 26, 1812.

Among the known forms of crystallized bodies, there is no one common to a greater number of substances than the regular octohedron, and no one in which a corresponding difficulty has occurred with regard to determining which modification of its form is to be considered as primitive; since in all these substances the tetrahedron appears to have equal claim to be received as the original from which all their other modifications are to be derived.

The relations of these solids to each other is most distinctly exhibited to those who are not much conversant' with crystallography, by assuming the tetrahedron as primitive, for this may immediately be converted into an octohedron by the removal of four smaller tetrahedrons from its solid angles. (Fig. 1.)

The substance which inost readily admits of division by fracture into these forms is fluor spar; and there is no diffieulty in obtaining a sufficient quantity for such experiments. But it is not, in fact, either the tetrahedron or the octohedron, which first presents itself as the apparent primitive form obtained by fracture.

If we form a plate of uniform thickness by two successive divisions of the spar, parallel to each other, we shall find the plate divisible into prismatic rods, the section of which is a rhomb of 70°32' and 109° 28' nearly; and if we again split these rods transversely, we shall obtain a number of regular acute rhomboids, all similar to each other, having their superficial angles 6o* and 120°, and presenting an appearance of primitive molecule, from which all the other modifications of such crystals might very simply be derived. And we find, moreover, that the whole mass of fluor might be divided into, and conceived to consist of, these acute rhomboids alone, which may be put together so as to fit each other without

any

intervening vacuity.

But, since the solid thus obtained (as represented fig. 2.) may be again split by natural fractures at right angles to its axis (fig. 3.), so that a regular tetrahedron may be detached

(, from each extremity, while the remaining portion assumes the form of a regular octohedron; and, since every rhomboid, that can be obtained, must admit of the same division into one octohedron and two tetrahedrons, the rhomboid can no longer be regarded as the primitive form; and since the parts into which it is divisible are dissimilar, we are left in doubt which of them is to have precedence as primitive.

In the examination of this question, whether we adopt the octohedron or the tetrahedron as the primitive form, since neither of them can fill space without leaving vacuities, there is a difficulty in conceiving any arrangement in which the particles will remain at rest: for, whether we suppose, with the Abbé Haüy, that the particles are tetrahedral with octohedral cavities, or, on the contrary, octohedral particles regularly arranged with tetrahedral cavities, in each case the mutual contact of adjacent particles is only at their edges; and

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