Page:Proceedings of the Royal Society of London Vol 1.djvu/466

Rh of adaptation. An instrument-maker, by employing a microscope, for the purpose of dividing with accuracy for several days together, is afterwards able to read without spectacles for a few Weeks, but his sight then gradually elongates, till he again has occasion to employ himself in dividing.

Two other cases are also mentioned, of long-sightedness reduced to vision at a moderate distance, both arising from disease, and both speedily relieved by evacuating remedies.

In this lecture the author undertakes to explain a difﬁculty that has occurred in crystallography, respecting the primitive molecule of those bodies that assume the octohedral and tetrahedral forms, when broken in the direction of their natural fractures.

The substance that he selects as most convenient for experiment is ﬂuor spar, which may very readily be divided into any number of acute rhomboids, having the angles of their surfaces 60° and 120°.

These might be regarded for all the practical purposes of crystallography as the integral molecule, and from thence all the other modiﬁcations of these solids might very simply be derived.

But it is observed, that each acute rhomboid thus obtained may be again split in a new direction at right angles to its axis, so that a tetrahedron may be detached from each extremity, leaving from between them a regular octohedron.

Consequently this rhomboid cannot be considered as the primitive, and we are left in doubt not only which to prefer of the two last-named solids, but even whether either of these can be primitive; since no possible arrangement of tetrahedra alone, or of octohedra alone, will ﬁll any space without leaving vacuities.

The author having observed that both these forms would result from the arrangement which spheres would most naturally assume by mutual attraction, proposes a theory founded on that observation, which he thinks is not liable to objection, and endeavours to extend this hypothesis, by showing that with some modiﬁcations a corresponding theory may be applied to other forms well known to crystallog'raphers.

With regard to the triangular arrangement of balls in a plane, and their tetrahedral grouping in solidity also, he ﬁnds that he has been anticipated by that universal genius Dr. Hooke: but he observes, that Dr. Hooke’s ideas upon this, as upon many other subjects, are but imperfectly developed; and that he takes no notice of the octohedral group, formed by placing four balls in a square, with one above and one beneath them. Accordingly, Dr. Hooke could know nothing of that which forms the principal novelty of the present observation, namely, that when a mass of spheres has been formed wholly according to the tdaugular or tetrahedral arrangement, then certain sections