1190 



ENTOPTIC PHENOMENA. 



[BOOK in. 



from the other eye : for as we have seen ( 726) light falling on 

 one retina leads to consensual narrowing of the pupil of the other 

 eye. Other changes or irregularities in the iris may be observed 

 by this method. 



Imperfections in the lens or in its capsule may also give rise 

 to entoptic images. Not uiifrequently a radiate figure corre- 

 sponding to the arrangement of the fibres of the lens makes its 

 appearance. 



The most common entoptic phenomena are those caused by the 

 presence of floating bodies in the vitreous humour, the so-called 

 muscce volitantes. These are readily seen when the eye is turned 

 towards a uniform surface, and are frequently very troublesome in 

 looking through a microscope. They assume the form of rows 

 and groups of beads, of single beads, of streaks, patches and 

 granules, and may be recognised by their almost continual move- 

 ment, especially when the head or eye is moved up and down. 

 When an attempt is made to fix the vision upon them they 

 immediately float away. 



Since the images on the retina are in these cases shadows and 

 since the strongest shadows are cast by parallel rays, the images 

 are best seen when the rays of light giving rise to the shadows on 

 the retina traverse the vitreous humour in parallel lines ; hence 

 the best illumination for examining the phenomena is one placed 

 in the principal anterior focus, the rays diverging from which fall 

 parallel on the retina ( 704, Fig. 135). The sharpness of the 

 images is also increased by using a small but bright source of 

 light, as by looking at a bright light through a small hole in a 

 screen. 



The sensations which these objects in the vitreous humour ex- 

 cite by means of the retinal images to which they give rise do not 



b' 



FlG. 144. DlAGKAJI TO ILLUSTKATE ENTOPTICAL IMAGES. 



