5-18 Royal Society : — 



structural stellate iigure, in the crystalline lens are placed methodi- 

 cally in the order in which they lie in the depths of the eye,' and 

 the especial manifestations which they severally yield, explained. 

 The combined effects of ocular chromatic aberration, inflection 

 at the edge of the iris, and the limbs of the stellate figure when we 

 look at thin objects, or black and white lines, especially if curved, 

 render some singular illusions, which are dissected. 



The method by the two sorts of pencils may be applied to test 

 the recent doctrine advanced by Stellweg, that the iris so lies on 

 the face of the crystalline lens that there is no posterior chamber 

 in the aqueous humour, and will probably be found to disprove it. 



A calculation is entered into to show that unless Dr. T. Young — 

 in estimating that the accommodation of the eye to focal distance 

 by means of an alteration in the length of the optic diameter, would 

 require a faculty of doing this to the extent of -f th of the whole, 

 taken when vision is suited to parallel rays, — exceeds the truth by 

 many times ; it must be easy to detect, by the parallax of the lenticular 

 corpuscles in a couple of pencils whose foci rest near them, how and 

 where the change is eifected. And then an argument is drawn, that 

 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 Strtictiire. 

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

 exists a lax, irregular, fibrous network, sprinying 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 uiaintained ia 1845 that the usual inuscaj volitantes are but 



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