2 Sir David Brewster on the Optical Phcenomena, Nature, 



appear to descend, as being situated before the pupil, or, at 

 least, before the place of intersection of the pencils*." 



Dr. Porlerfield, who has given a very inaccurate drawing 

 of the filamentous Muscle, considers them as produced by dia- 

 phancus particles and filaments, that swim in the aqueous hu- 

 mour before the crystalline ; and he regards the distinct pic- 

 tures of them upon the retina o\' Iong-si;j:,hted persons, as pro- 

 duced by the rays which pass through the dense particles, 

 liavin"- suffered a greater retraction than those which pass by 

 them, so as to be converged to foci upon die retinaf. 



Tiie latest writer on this subject, Mr. Mackenzie of Glas- 

 gow, describes ihe Musccc as resembling ?«/«?</f, twisted, semi- 

 transparent tubes, partially filled with globules, which some- 

 times appear in motion : while another set are more opake, 

 or perfectly dark, and follow the motions of the eye. The 

 latter he considers as " of a more dangerous character than 

 the former, and as occasioned, generally, by a partial insen- 

 sibility of the retina," either from the pressure of some "irre- 

 gular projecting point or points of the choroid, or from some 

 other cause." ' Mr. Mackenzie regards the globules within 

 the semi-transparent tubes, as probably '■'■ blood passing through 

 the vessels of the retina, or of the vitreous humour;" and he 

 remarks, "that neither these semi-transparent tubes themselves, 

 nor any of the filamentous Musctv, or black spots (which are 

 so frequently complained of), possess any real motion, inde- 

 pendent of the general motion of the eyeball ;" and iience he 

 concludes that they " must be referred either to the retina 

 itself — including, of course, the three laminae of which it is 

 composed,— or to the choroid coat." "The probability is," 

 he adds, " that the semi-transparent Mnsc(C, of a tubular form, 

 are owing to a dilatation of the branches of the arteria cen- 

 tralis retina: X" 



Such was the state of our information on the subject of 

 Musca; volitantes, when my attention was specially directed to 

 it, in consequence of finding in my own eye a good example of 

 the phasnomenon ; and, having carefully investigated the facts 

 as observed by other persons in their own eyes, I trust I shall 

 be able to lay before the Society a correct description, and a 

 satisfactorv explanation, of the general phaenomena. 



Although the bodies which are within the eyeball, and give 

 rise to the phaenomena under consideration, are often seen 

 under ordinary circumstances, yet, in order to see them with 



• Smith's Optics, vol. ii. Rem. p. 5. 



\ Treatise on tlie Eje, vol.ii. p. 74-80. 



* Practical Treatise on the Diseases of the Eye, 1830, pp. 748,750. 



