CAUSES OF EEEOES. 777 



with reflected light their rim appears white. Pressure under a 

 glass cover may cause them to assume very irregular shapes, but pos- 

 sessing the same properties in their margin or outline in their be- 

 haviour with the light. It is also necessary to become familiar with 

 the appearances of the lowest forms of animal and vegetable life, such, 

 for instance, as the common forms of infusoria ; also the yeast, and such 

 like plants ; and the different forms of mould. A peculiar motion, 

 known as " Brownian motion," is also a phenomenon which must be 

 recognised. It is peculiar to all very small particles when they float 

 in a very thin fluid medium. It is well seen in the iine granules of 

 milk when mixed with water, and in the milky juices of plants. 

 A magnifying power of 400 or 500 diameters is the best for this ob- 

 servation. The eye itself is a source of deception, inasmuch as the 

 phenomena known as " muscae volitantes " appear as if they were 

 objects seen by the microscope. These are described as follows by 

 Dr. W. Mackenzie in his Treatise on the Eye : — 



" The vision of objects on the surface or in the interior of the eye 

 has attracted attention, chiefly in relation to a symptom to which the 

 name of musam volitantes has been given. Any spectrum or visual 

 appearance which is apt to impose on the mind, and lead one to think 

 that flies are moving before the eye, is called a musca voUtans (fig. 

 952). 



" The condition comprehends those sensations which arise from — 

 1. The layer of mucus and tears on the surface of the cornea ; 2. 

 Corpuscles between the external surface of the cornea and the focal 

 centre of the eye ; 3. Corpuscles between the focal centre of the eye 

 and the sensitive layer of the retina. 



" In hanging the head over the microscope, especially if one is 

 affected with catarrh at the time, the globules, by gravitating to the 

 centre of the cornea, not unfreqiiently appear to the observer so as to 

 impede his view of the object, till by the act of nictitation he clears 

 them away. In telescopic observations, also, the muco-lacrymal 

 spectrum is apt to prove a source of annoyance. Thus, in looking at 

 the sun through a tinted glass, the observer may be unable to dis- 

 tinguish the spots on that body, being perplexed by what seems the 

 reflection of some part of his own eye interposed between it and the 

 sun. This is caused by the layer of mucus and tears on the surface 

 of the cornea. 



" If one looks at the flame of a dandle two or three feet distant, 

 or at the sky, through a hole made in a blackened card with the point 

 of a fine needle, or through a convergent lens of short focus, such as 

 the eye-glass of a compound microscope, on steadily regarding the 

 luminous field presented to view, four sets of spectra wUl be seen 

 (fig. 952), independent of the muco-lacrymal spectrum. The most 

 remarkable appears nearest to the eye, and consists of twisted strings 



