AND LOCALITY OF MUSCLE VOLITANTES. 381 



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 microscope 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 phenomena 

 as the analogous bodies within the eyeball. 



Hence it follows, that the filaments and spherical particles, whose diffracted 

 shadows have four or five different sizes, have the same magnitude, and are placed 

 at four or five different 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. These various bodies, though they change 

 their place, still preserve their general distance from the retina, thus clearly indi- 

 cating that the vitreous humour is composed of cells within which the filaments 

 and muscce are lodged. That they do not exist in the aqueous humour is 

 very obvious, because if they did, they would either rise to the top or sink 

 to the bottom of the aqueous chamber when the eyeball was at rest, and thus 

 withdraw themselves entirely from the field of view, which they never do. 



In order to obtain farther information respecting these muscte, I fixed the eyeball 

 in different positions, and looking at a sheet of white paper, I marked upon it the va- 

 rious positions on the paper where the Musca rested. It never withdrew itself from 

 the field of view, and suffered no sensible change in its size ; but it rested in posi- 

 tions at different distances from the axis of vision. In one position of the head, 

 I could bring the musca into the optic axis so as to obtain the most perfect vision 

 of it, but in all other positions of the head, it rested at a distance from the optic 

 axis ; though in these it could, by a toss of the head, be made to cross the axis 

 of vision. In making these experiments, we must recollect that, as the musca is 

 generally seen by oblique vision, it will very frequently disappear, though it has 

 not withdrawn itself from the field of view. In all positions of the head, the 

 musca, appears to descend, so that it must actually ascend in the vitreous humour, 

 and be specifically lighter. 



Now, it is obvious, that, if we determine the visible position of the Musca 

 when at rest in different positions of the head, we determine the direction of lines 

 passing from the centre of visible direction through the points in the vitreous hu- 

 mour where the musca rested, and thus obtain a general notion of the form of the 

 cell in which it is contained. But Ave may go still farther, and determine with 

 considerable accuracy the diameter of the Musca or its filaments, and also then- 

 distance from the retina, and thus obtain a knowledge of its locality, and of the 

 form of the cavity by which its excursions are limited. 



In order to do this, I place before the eye two bright sources of light, so as 

 to obtain from them, by the method already described, two divergent beams of 

 light, and I thus obtain double images on the retina of all objects placed within 

 the eyeball. The filaments or Musccs in the anterior part of the vitreous humour 



VOL. XV. PART III. 5 K 



