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XXVI. On the Optical Phenomena, Nature, and Locality of Muscce Volitantes; 

 with Observations on the Structure of the Vitreous Humour, and on the Vision 

 of Objects placed tvithin the Eye. By Sir DAVID BKEWSTEE, K.H.D. C.L., F.R. S., 

 and V.P.R.S. Edin. ***** 



(Read, 6th March 1843.) 



ALTHOUGH some of the phenomena of Muscce wlitantes may be seen by per- 

 sons of all ages, and with the best eyes ; and though those which are more pecu- 

 liarly entitled to the name are exceedingly common beyond the middle period of 

 life ; yet no account has been given of them that has even the slightest pretension 

 to accuracy. M. DE LA HIRE, in his Differens accidens de la Vve, describes these 

 Muscce as of two kinds ; some permanent and fixed, which he ascribes to small 

 drops of extravasated blood upon the retina; and others, as flying about, and 

 changing their place, even though the eye be fixed. The first kind, he describes 

 as like a dark spot upon a white ground ; and the second, as like the knots of a 

 deal board. Some parts of them, he says, are very clear, and surrounded with 

 dark threads, and are accompanied with long fillets of irregular shapes, which are 

 bright in the middle, and terminated on each side by parallel black threads. 



In order to account for these knots and irregular fillets, BE LA HIRE sup- 

 poses that " the aqueous humour is sometimes troubled with some little mothery 

 ropy substance, some parts of which, by the figures of their little surfaces, or by 

 refractive powers different from the humour itself, may cast then" distinct images 

 upon the retina. He supposes them in the aqueous humour rather than in the 

 vitreous, because of its greater fluidity for a freedom of descent, and because they 

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

 the place of intersection of the pencils." * 



Dr PORTERFIELD, who has given a very inaccurate drawing of the filamentous 

 musc(B, considers them as produced by diaphanous particles and filaments, that 

 swim in the aqueous humour before the crystalline ; and he regards the distinct 

 pictures of them upon the retina of long-sighted persons, as produced by the rays 

 which pass through the dense particles, having suffered a greater refraction than 

 those which pass by them, so as to be converged to foci upon the retina.f 



The latest writer on this subject, Mr MACKENZIE of Glasgow, describes the 

 muscte as resembling minute, twisted, semi-transparent tubes, partially filled with 



* Smith's Optics, vol. ii. Rcm. p. 5. t Treatise on the Eye, vol. ii. p. 74-80. 



VOL. XV. PART III. 5 I 



