ADVANCED EXPERIMENTAL PHYSIOLOGY 263 



No difficulty will be experienced in recognising 'muscae volitantes.' 

 These will appear as small particles or threads which appear to move 

 away rapidly when the gaze is directed at them. When the gaze is 

 fixed, as by a mark on the white paper, they are still seen to move 

 slowly downwards. This implies that actually their shadows are 

 moving slowly upwards, and that the objects themselves are similarly 

 slowly ascending in the vitreous. 



If, whilst gazing at some distinct cluster of muscae volitantes, the eye 

 move upwards, the cluster will appear to move upwards too. This 

 actually means that the shadow of the cluster is moving downwards 

 on the retina. If the card be moved downwards the same result, 

 as far as the shadows are concerned, will occur. From this it may be 

 inferred that the objects producing the shadow are behind the nodal 

 point (situated in the crystalline lens), and therefore, if the movement 

 of shadow be appreciable, on the vitreous. 



Objects in front of the nodal point, such as impurities on the cornea, 

 would appear to move upwards when the gaze is directed downwards, 

 and conversely. 



CHAPTER LXXIIL 



THE INSTRUMENTS USED IN THE CLINICAL INVESTIGATION OF 



THE EYE. 



The Measurement of the Field of Vision.- If the eye be fixedly 

 directed to some particular point it is possible to see objects at some 

 distance from this point. The area in which objects can be seen with the 

 eye thus fixated is spoken of as the field of vision. With the head fixed 

 and the eye allowed to move as far as possible in any direction a much 

 larger area can be viewed. This area is spoken of as the field of regard. 



Though fairly satisfactory results can be obtained by using a com- 

 paratively simple form of apparatus called a campimeter, it is customary 

 to employ an instrument called a perimeter to obtain accurate details of 

 the extent of the field of vision. 



The perimeter (see Fig. 219), consists of a quadrant upon which a 

 white spot can be moved, and this quadrant can be revolved about a line 

 continuous with the optic axis. At K is the chin rest, double, so as to 

 enable either eye to be adjusted against 0. The subject having taken 



