The Microscope. 361 



It is a well-known fact that in the vitreous humor, a semi-gel- 

 atinous mass, filling up the greater part of the posterior portion 

 of the eye, there are frequently floating small particles, which 

 are known to oculists as " muscse volitantes." 



The origin of this imperfection does not concern us here ; it 

 is only of interest to know that, though existing practically in 

 almost every eye, ordinarily they are not visible, just as the 

 dust in a room is not visible when the light enters through the 

 widely open windows, and is equally diffused throughout the 

 room ; but if a beam of sunlight enters through a small opening 

 in the shutter, then innumerable objects are seen dancing in the 

 tract of the beam, not only reflecting the rays, but casting 

 shadows behind them. So it is possible for almost any indivi- 

 dual to see the small particles in the vitreous humor of his own 

 eye. For this purpose it is necessary, however, to allow the 

 light to enter it, not through the widely opened pupil, but 

 through a small aperture (for example, by pricking a pin hole 

 in a card and looking through at the sun, or at some ©ther 

 source of very bright light.) When this is done, the beam 

 illuminates the particles which may be in the vitreous humor so 

 strongly that they throw a shadow on the retina, and thus be- 

 come visible. 



The form of these particles varies considerably. Usually 

 they are in the form of fine dots, more or less well defined, and 

 occasionally these dots have such sharp edges and are so well 

 marked or perfectly stationary that they can well be confused 

 with certain microscopical objects which we shall consider 

 presently. 



Again several of these dots are joined in the form of a chain, 

 or still again, several chains or particles may move together. 

 These are the appearances most frequently observed. 



Now as to the application of this pathological, or, practically, 

 physiological, fact to the use of the microscope. Let us see 

 what the connection is. We have the same optical conditions, 

 especially when high powers are used. The ray of light is a 

 small one, as it comes through the small opening in the dia- 

 phragm, and must necessarily be as strong as possible. Then 

 again the reason for using high powers is to observe more 

 exactly exceedingly fine objects, and one of the branches of 

 nucroscopical work in which these particles in the vitreous are 



