378 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA, NATURE, 



globules, which sometimes appear in motion ; while another set are more opaque, 

 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 insensibility of the retina," either from the pressure of some " irregular 

 projecting point or points of the choroid, or from some other cause." Mr MAC- 

 KENZIE 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 re- 

 marks, " that neither these semi-transparent tubes themselves, nor any of the fila- 

 mentous muscse, or black spots (which are so frequently complained of), possess 

 any real motion, independent of the general motion of the eyeball ;" and hence 

 he concludes that they " must be referred either to the retina itself including, 

 of course, the three laminse of which it is composed, or to the choroid coat." 

 " The probability is," he adds, " that the semi-transparent muscce, of a tubular 

 form, are owing to a dilatation of the branches of the arteria centralis retina.*' 1 ' 1 



Such was the state of our information on the subject of Muscce volitantes, 

 when my attention was specially directed to it, in consequence of finding in my 

 own eye a good example of the phenomenon ; 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 satisfactory explanation, of 

 the general phenomena. 



Although the bodies which are within the eyeball, and give rise to the phe- 

 nomena under consideration, are often seen under ordinary circumstances, yet, in 

 order to see them with distinctness, we must look at the sky, or a luminous ob- 

 ject, either through a very minute aperture, or, Avhen the light is limited or feeble, 

 through a lens or microscopic doublet, of very short focus, held close to the eye. 

 By this means, we shall observe a luminous ground, covered, more or less, with 

 transparent filaments or tubes, transparent circles, exceedingly minute, and (when 

 they do exist) with Musca;, or black spots like flies. 



In examining the transparent filaments, I have observed them of four or five 

 different sizes, the smallest of which are the most distinct. These distinct fila- 

 ments are bounded by two sharp black lines, and the space between them is more 

 luminous than the general ground on which they are seen. In the larger fila- 

 ments, the black lines are coloured at their edges, and, on the outside of each of 

 them, are one or more coloured fringes. 



The minute transparent circles, when smallest, have a luminous centre, with 

 a sharp black circle round it. In the larger ones, this circle is coloured at its 

 edges ; and, on the outside of it, are one or more circular coloured fringes. These 

 spherical bodies sometimes exist singly, and sometimes in groups, partly connected 

 by small filaments, and partly by an invisible film, to which they seem attached. 



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



