446 



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. 



The Bakerian Lecture, On the elementary Particles of certain Crystals. 

 By William Hyde Wollaston, M.D. Sec. R.S. Read November 

 26, 1812. [_Phil Trans. 1813,;?. 51.] 



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 crystal- 

 lography as the integral molecule, and from thence all the other mo- 

 difications 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 be- 

 tween 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 corre- 

 sponding theory may be applied to other forms well known to cry- 

 stallographers. 



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 octo- 

 hedral 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 accord- 

 ing to the triangular or tetrahedral arrangement, then certain sections 



