607 



the accommodation is by change in the form of the lens, producing 

 a minute movement of its anterior face, which it is thought may 

 be detected by the said method. 



The want of symmetry in ocular refractions is glanced at, and a 

 nebulous scattering of light in the eye, hereafter found to be the 

 cause of a singular supplementary version of Purkinje's vascular 

 phantom. 



SECTION IV. Apparitions from the Vitreous Humour, applied to 

 explain its Structure. 



It can be observed that, in the posterior chamber of the eye there 

 exists a lax, irregular, fibrous network, springing from the hyaloid 

 membrane, but spanning the crystalline lens, without attachment 

 to its capsule, occupying principally the peripheral portion of the 

 cavity, but spreading as one structure into its interior, towards an 

 ever-lessening number of leading fibres. The whole system is of 

 less specific gravity than the vitreous fluid, either of itself, or by 

 being the framework of membrane, in more or less of its extent. 

 The fibres are constituted entirely of rows of beads, which are 

 round, or nearly so, transparent, and of greater refractive power 

 than the fluid, and joined by passing into one another by small 

 portions of their surface. The dynamical and optical considerations 

 upon which these conclusions depend, are very carefully entered 

 into, and the nicer points illustrated with appropriate drawings *. 



SECTION V. Apparitions from, or from behind the Retina ; with 

 Corollaries. 



The next object for study behind those in the vitreous, are the 

 vasa centralia retinae, which are imbedded in the substance of the 



* The 36th vol. pp. 97-104, of the Lond. Med. Gazette, is quoted to 

 show that the writer maintained in 1845 that the usual muscae volitantes are but 

 apparitions of portions of the essential structure of the vitreous body, and that 

 he then fundamentally and clearly enunciated the view now more particularly 

 developed. Other writers have regarded these as remnants of the foetal eye, or 

 as pathological fragments, floating freely, or in loculi of the vitreous humour ; 

 for the most part differing very widely from the reasonings and conclusions in 

 this essay. The writings of Brewster, Bonders, Doncan, &c., are referred to. 



