CHAP, in.] SIGHT. 875 



on the retina, the pin-hole should occupy the position of the 

 principal anterior focus), the dark circle which bounds the field 

 of vision is the image caused by the shadow of the margin not 

 as might at first be supposed of the pin-hole but of the iris. 

 This is at once shewn by the changes which it can be made to 

 undergo, while the pin-hole remains motionless, by alternately 

 closing and opening the other eye ; the field of vision of the 

 eye which is looking through the pin-hole may be observed to 

 contract when light enters, and to expand when the light is 

 shut off from the other eye ; for as we have seen ( 539) 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 unfrequently 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 recognized by 

 their almost continual movement, 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 phenom- 

 ena is one placed in the principal anterior focus, the rays 

 diverging from which fall parallel on the retina ( 526, Fig. 

 140). 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 

 excite by means of the retinal images to which they give rise 

 do not tell us that the objects are in the vitreous humour. As 

 we shall see we refer all affections of the retina, all visual sen- 

 sations to some changes in the external world ; and if we trusted 

 to our sensations alone in the cases of these entoptic phenom- 

 ena we should suppose that the causes existed outside ourselves. 

 It is only by means of inferences drawn from the features and 

 behaviour of the sensations that we arrive at the conclusion 

 that the causes lie in the vitreous humour. 



The accompanying diagram (Fig. 149) illustrates how the 



