THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



JANUARY 1848. 



A 



On the Optical Phccnomena, Nature, and Locality of 

 Muscae volitantes ; mth Observations on the Stmctnre of 

 the Vitreous Humour, and on the Vision of Objects placed 

 •within the Eye. By Sir David Brewster, K.H., D.C.L.^ 

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



LTHOUGH some of the phaenonieiia of M?csc^ voli- 

 tantes may be seen by persons of all ages, and with the 

 best eyes; and though those which are more peculiarly en- 

 titled 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 Vue, 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 frst 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, De 

 la Hire supposes that "the aqueous humour is sometimes trou- 

 bled with some little mothery roi)y substance, some parts of 

 which, by the figures of their little surfaces, or by refractive 

 powers different from the humour itself, may cast their distinct 

 imaoes 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 



• From the Transactions of the Roval Society of Eclinlinrgh, vol. xv. 

 p. 377. Read March 6. 18 43. 



Phil. Mas. S. 3. Vol. 32. No. 212. Jan. 184.8. B 



