8 Sir David BiewsLer un the Optical PJuvnomenn, Natur 



invisible, unless it be of such a size, and so near the retina, as 

 to show itself by its ordinary shadow. But in whatever way 

 the image of the object is formed, the mind takes cognizance 

 of it, or gives it an external locality, by means of the same 

 law of visible direction which regulates the vision of objects 

 placed without the eyeball. 



Wiiile these results exhibit the true physical cause of all 

 the optical phasnomena and limited movements of the filaments 

 and Musccc, they lead also to some important and useful con- 

 clusions of a more general nature. It had been conjectured 

 that the vitreous humour of animals was inclosed in separate 

 bags or cells connected with the hyaloid membrane by which 

 the vitreous mass is enveloped. The preceding experiments 

 not only appear to demonstrate that this is the structure of the 

 vitreous humour in man, but to show that there are at least 

 four or Jive cells between the retina and the posterior surface 

 of the crystalline lens. The limited motion of the Muscce in- 

 dicates that the cell in which they float is of very limited ex- 

 tent. When the vertical diameter of the eyeball, in its natural 

 position, is placed, by the inclination of the head, 30" to the 

 ri'dit-hand of a vertical line, and the optic axis of the eye di- 

 rected 20° below a horizontal line, the Miisca is seen along 

 the optic axis, and consequently in the most perfect manner. 

 One point of its cell must therefore touch the optical axis. 



I have endeavoured, with the assistance of my eminent col- 

 lea'^ue Dr. Reid, to discover cells in the vitreous humour of 

 quadrupeds and fishes by the aitl of the microscope and other 

 means, but we have not succeeded : and unless some chemical 

 substance shall be found which acts differently upon the albu- 

 minous fluid and the membranous septa, it is not likely that 

 they will be otherwise rendered visible*. 



Mr. Ware, in a paper on the Miisc^ volitantes of 7iervous 

 personsf, describes some as "globules twisted together, and 

 others as like the flue that is swept from bed-rooms," and he 

 considers it " probable that they depend on a steady pressure 

 on one or more minute points of the retina which are situated 

 near the axis of vision J." In the cases described by Mr. Ware, 

 the Muscce were liable to great and sudden changes in inten- 

 sity and number, particularly from causes affecting the nervous 



• The vitreous humour, vvlien slowly dried, either by itself, or along 

 with parts of the septa in which it may be contained, shoots into beautiful 

 crystalline ramifications proceeding from the four angles of a quadrilateral 

 crystal. Thin six-sided plates frequently occur, Imt they seem to exercise 

 no action upon polarized light, probably on account of their thinness. The 

 same effects were produced when the vitreous liumour from a fresh eye was 

 well-washed in distilled water. 



t Medico-ChirurgicHl Trans., 1814, vol. v. p. 255. : Ibid. p. 266. 



