THE SENSE OE VISION. 321 



is visible without the use of the focal illumination, some inconvenience may 

 thus be caused in using a vertical microscope. 



A study of the motions of the m/uscce volitantes makes it evident that the 

 phenomenon is due to small bodies floating in a liquid medium of a little 

 greater specific gravity than themselves. Their movements are chiefly in 

 planes perpendicular to the axis of vision, for when the eye is directed verti- 

 cally upward they move as usual through the field of vision without increasing 

 the distance from the retina. They are generally supposed to be the remains 

 of the embyronic structure of the vitreous body — i. e. portions of the cells and 

 fibres which have not undergone complete mucous transformation. 



In addition to these floating opacities in the vitreous body various other 

 defects in the transparent media of the eye may be revealed by the method of 

 focal illumination. Among these may be mentioned spots and stripes due to 

 irregularities in the lens or its capsule, and radiating lines indicating the stel- 

 late structure of the lens. 



Retinal Vessels. — Owing to the fact that the blood-vessels ramify near the 

 anterior surface of the retina, while those structures which are sensitive to light 

 constitute the posterior layer of that organ, it is evident that light entering the 

 eye will cast a shadow of the vessels on the light-perceiving elements of the 

 retina. Since, however, the diameter of the largest blood-vessels is not more 

 than one-sixth of the thickness of the retina, and the diameter of the pupil is 

 one-fourth or one-fifth of the distance from the iris to the retina, it is evident 

 that when the eye is directed to the sky or other broad illuminated surfaces it 

 is only the penumbra of the vessels that will reach the rods and cones, the umbra 

 terminating conically somewhere in the thickness of the retina. But if light 

 is allowed to enter the eye through a pin-hole in a card held a short distance 

 from the cornea, as in the above-described method of focal illumination, a 

 sharply defined shadow of the vessels will be thrown on the rods and cones. 

 Yet under these conditions the retinal vessels are not rendered visible unless 

 the perforated card is moved rapidly to and fro, so as to throw the shadow 

 continually on to fresh portions of the retinal surface. When this is done the 

 vessels appear, ramifying usually as dark lines on a lighter background, but 

 the dark lines are sometimes bordered by bright edges. It will be observed 

 that those vessels appear most distinctly the course of which is at right angles 

 to the direction in which the card is moved. Hence in order to see all the 

 vessels with equal distinctness it is best to move the card rapidly in a circle 

 the diameter of which should not exceed that of the pupil. In this manner 

 the distribution of the vessels in one's own retina may be accurately observed, 

 and in many cases the position of the fovea centralis may be determined l>\ the 

 absence of vessels from that portion of the macula lutea. 



The retinal vessels may also be made visible in several other ways — e. </., 

 1. By directing the eye toward a dark background and moving a candle to and 

 fro in front of the eye, but below or to one side of the line of vision. 2. By 

 concentrating a strong light by means of a lens of short focus upon a point 

 of the sclerotic as distant as possible from the cornea. By either of these 

 methods a small image of the external source of light is formed upon the 



v,„ TT — 21 



