4 Sir David Brewster on the Optical Phccnomena, Nature, 



All the bodies which we have now described have two dif- 

 ferent motions ; one arising from the motion of the head or 

 eyeball, and the other when the eyeball is absolutely fixed. 

 By a toss of the head they are thrown into difl'erent absolute 

 and relative positions, sometimes ascending and descending in 

 succession, sometimes oscillating between two limits, and ge- 

 nerally with different velocities. When the eye is first applied 

 to the lens or aperture, the field of view is tolerably free of 

 these moving bodies ; but the light seems to stir them up, as 

 it were, anti, to a certain extent, the longer we view them, the 

 more numerous do they become. 



If the centre of motion of the eyeball coincides with the 

 centre of visible direction, the Muscce will ascend when the eye 

 looks npxiiard, and vice versdf whether they are placed before 

 or behind that common centre. If the eyeball remains fixed, 

 the Muscre \n front of the above centre will have the direction 

 of their real and apparent motions the same, and those behind 

 that centre will have these two directions different. Hence 

 the appearance of two opposite currents when the eyeball is 

 turned quickly from one extreme of its range, either vertically 

 or horizontally, to its mean position; and so ra[)id is their 

 motion through the luminous field, that it seems covered with 

 continuous lines parallel to the direction in which the eyeball 

 has been moved, — an effect arising from the duration of the 

 impression of light upon the retina. 



If we mark individual filaments, or groups, or knots, we 

 shall find that they change their shapes, one part of a filament 

 doubling itself over another, and again resinning its elongated 

 form. The minute spherical bodies separate and approach 

 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 necessarily arises from the 

 bending of the filament, and from the varying oblicjuity of 

 different parts of it owing to its change of form or place, we 

 are not entitled, from this apparent motion, 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 phaenomena 

 of the real Muscre, 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 s|)herical specks, and something 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. 



