370 PEACTICAL PHYSIOLOGY 



rubbing the eyes. These are all due to the condition of the corneal 

 surface, and have been more properly called ' pseudentoptic ' phenomena. 



(b) Dark specks or irregularly stellate figures may be seen, depending 

 upon imperfections in the lens or its capsule. 



(c) Muscae Volitantes. These appear as moniliform threads, clusters of 

 bright or dark circles, and are referable to imperfections in the vitreous. 



EXPERIMENT. Place a card which is pierced by a pinhole a little 

 more than a centimetre from the eye (i.e. in the position of the principal 

 anterior focus of the * reduced ' eye). Look at an evenly but brightly 

 illuminated surface beyond, as a sheet of thin white paper held in front 

 of a lamp. The rays of light falling on the retina are now approxi- 

 mately parallel, and any shadows that form in consequence of imper- 

 fections in the refracting media are rendered more distinct. Notice 

 any of such shadows that may be received by blinking, due to im- 

 perfections in the cornea or any comparatively fixed figure due to 

 imperfections in the crystalline lens. These may be practically absent. 

 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. 



CHAPTEE XXXVI. 

 SENSATIONS OF LIGHT AND COLOUR. 



MANY theories have been advanced to explain the phenomena con- 

 nected with colour vision. The most important of these theories are 

 those connected with the names of Young-Helmholtz and Hering. 





