AND LOCALITY OF MUSCLE VOLITANTES. 379 



They sometimes adhere to the outside of the filaments, and very frequently oc- 

 cur mithin the filaments, so as to prove that these filaments are tubular. These 

 spherical bodies have, like the filaments, four or five different sizes. 



In making observations on these spherical bodies, the observer will sometimes 

 see luminous spots pass through the field ; but as these arise from the state of 

 the lubricating fluid on the outside of the cornea, they have no connection with 

 the phenomena under our consideration. 



The transparent filaments, already described, are seldom seen single. Two 

 or three are united, like threads crossing one another ; and sometimes a great 

 number are united, like a loose heap of thread, in which case, obscure spots ap- 

 pear at the places where the crossings of the filaments are most numerous. 



In some cases, a single long filament is once or twice doubled up upon itself, 

 and sometimes a knot is, or appears to be, tied upon it, consisting of several folds, 

 as it were, of the filament. This knot has several very dark spots at the places 

 where the different portions of the filament are in contact ; and this accumula- 

 tion, as it were, of black specks, constitutes the real muscce. In many, indeed in 

 almost all of these muscte, when distinct, a little bright yellow light accompanies 

 the black specks. 



All the bodies which we have now described have two different 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 different abso- 

 lute and relative positions, sometimes ascending and descending in succession, 

 sometimes oscillating between two limits, and generally 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, and, 

 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 upward, and vice versa, 

 whether they are placed before or behind that common centre. If the eyeball 

 remains fixed, the Musca; in 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 verti- 

 cally or horizontally, to its mean position ; and so rapid 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 du- 

 ration 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 

 resuming its elongated form. The minute spherical bodies separate and approach 



