380 



SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA, NATURE, 



one another ; but I have not been able to satisfy myself that those within the 

 tubular filaments change their place. They often appear to do so ; but as this 

 may arise from the bending of the filament, or from the varying obliquity of diffe- 

 rent parts of it arising from its change of form or place, we are not entitled to 

 consider them as moveable within the tube. It is certain, however, that they 

 have no progressive motion, as supposed by Mr Mackenzie. 



In order to obtain a correct knowledge of the phenomena of the real Miiscte, 

 I confined my attention to one in my own eye, of which I first made a drawing 

 in October 1838. It is represented in the annexed figure, and consists of four fila- 



ments, ABC, BDE, FGH, and AK. Between BC and BDE there is a sort of 

 transparent web containing a great number of minute spherical specks, and some- 

 thing similar, though less extensive, below FGH. The real Musca exists at A, 

 and has obviously been produced by the accidental overlapping of the different 

 filaments which are united with it. In four and a half years, the Musca at N has 

 perceptibly increased in size, and the length of the associated filaments has di- 

 minished. It is distinctly seen without any of its accompaniments in ordinary 

 light, but is, in no respects, injurious to vision, as it is never stationary in the 

 axis of the eye. When seen by means of the lens, the long branch FGH takes 

 various positions, sometimes falling below the knot or musca A, and sometimes 

 crossing the main branch AB, below B. The branch BDE has often a loop at D, 

 and FGH another at G. 



Having had occasion to study the phenomena of the diffraction of light, as pro- 

 duced by transparent fibres and films of different forms, I could not fail to observe 

 that the phenomena above described were the shadows formed on the retina by 

 divergent light passing by and through transparent filaments and particles placed 

 within the eyeball. They are indeed perfectly identical, and may be accurately 

 imitated in various ways. If we crush a crystalline lens in distilled water, or 



