6 Sir David Brewster on the Optical Phcenomena, Nature, 



These various bodies, though they change their place, still 

 preserve their general distance from the retina, thus clearly 

 indicating that the vitreous humour is composed of cells within 

 which the filaments and Musccv 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 further information respecting these 

 Muscat, I fixed the eyeball in different positions*, and looking 

 at a sheet of white paper, I marked upon it the various posi- 

 tions on the paper where the Musca rested. It never with- 

 drew itself from the field of view, and suffered no sensible 

 change in its size ; but it rested in positions at different di- 

 stances 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 Miisca 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 frequently disappear, though it has not withdrawn itself from 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 ap- 

 pears 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 humour 

 where the Musca rested, and thus obtain a general notion of 

 \.\\efo7m of the cell in which it is contained. But we may go 

 still further, and determine with considerable accuracy the 

 diameter of the Musca or its filaments, and also their 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 

 oflight, soas 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 })laced within the 

 eyeball. The filaments or Musae in the anterior part of the 

 vitreous humour will have their double images very distant: 



• In order to do this, the observer must place himself on his back, on 

 his fare, on his ri^ht side or on his left side, or he may place the vertical 

 axis of his eyeball at any angle to a vertical line. 



