and Locality q/'MuiCae voHtantes. 5 



In four and a half years, the Miisca at N has perceptibly in- 

 creased in size, and the length of the associated filaments has 

 diminisheil. It is distinctly seen without any of its acconi- 



panimeiits 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 len^^, 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 phaenomena of the dif- 

 fraction of light, as produced by transparent fibres and films of 

 different forms, I could not fail to observe that the phaenomena 

 above described were the shadows formed on the retina by 

 divergent light passing liy 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 

 macerate some very thin laminae of it, and dry a drop of the 

 fluid on a piece of glass, we shall perceive, with a fine micro- 

 scope a little out of focus, or with an ill-adjusted illuminating 

 apparatus, a number of minute fibres, single and in groups, 

 and knots, with minute spherical particles, which display the 

 very same phaenomena o\' diffraction as the analogous bodies 

 wilfiin the eyeball. 



Hence it follows, that the fdaments and spherical particles, 

 whose diffracted shadows have four or five different sizes, 

 have tile same magnitude, and are placed at four or five tliffer-i 

 ent distances from the retina ; those which give the sharp, 

 black, and minute shadows, being placed near the retina, and 

 those which are large and ill- defined at a distance from it. 



