18 SIXTH REPORT. — 183G. 



— and that, in consequence of this property, the lens gradually In- 

 creases in bulk, and becomes more globular, till the capsule bursts 

 with the expansive force of the overgrown lens. That the reaction 

 of the elastic capsule contributes to modify the polarizing structure 

 of the interior mass, cannot admit of a doubt, as it is easy tc prove 

 that that structure is altered by mechanical pressure ; but I cannot 

 conceive how such a reaction could create a new negative structure 

 between two positive ones, and produce the other pheenoraena which 

 I have described. I have been led therefore to the opinion, that 

 there is in the crystalline lens the germ of the perfect structure, or 

 rather the capability of its being developed by the absorption of the 

 aqueous humour ; that this perfect structure is not produced till the 

 animal frame is completely formed ; and that when it begins to 

 decay the lens changes its density and its focal length, and some- 

 times degenerates into that state which is characterized by hard and 

 soft cataract. 



The results of which I have now given an exceedingly brief notice, 

 appear to me to aiford a satisfactory explanation of those changes 

 in the lens which terminate in cataract, a disease which seems to be 

 more prevalent than in former times. Accidental circumstances 

 have led me to study the progress of this disease in one peculiar 

 case, in which it was arrested and cured ; and I am sanguine in the 

 hope that a rational method of preventing, and even of stopping the 

 progress of this alarming disease, before the laminae of the lens have 

 been greatly separated or decomposed, may be deduced from the 

 preceding observations. 



As the experiments, however, and views upon which this ex- 

 pectation is founded, are more of a physiological than of a physical 

 nature, 1 am desirous of submitting an account of them to the Me- 

 dical Section, that they may undergo that strict examination which 

 they could receive only from the experience and science of that di- 

 «tinguished body. 



On the Lmvs of Double Refraction in Quartz. By J. M'Cullagh, 

 Fellow of Ti-inity College, Dublin. 



Among the desiderata of optical science, one of the most remark- 

 able is a mechanical theory of the laws of double refraction in quartz, 

 or rock-crystal. These laws, which, as far as we know, are pecu- 

 liar to that crystal, were made out by the successive labours of Ara- 

 go, Biot, Fresnel, and Airy ; of whose researches a full account 

 has been given in the Report on Physical Optics, drawn up for the 

 Association by Professor Lloyd*. But the laws so discovered were 

 merely isolated facts ; no connexion had been traced amongst them, 

 if we except Fresnel's beautiful explanation of the rotatory phaeno- 

 mena. It was the object of Mr. M'CuUagh's communication to 



* Reports of the British Association for the Advancement of Science, vol. iii. 

 p. 405—409. " 



