130 VETERINAEY PHYSIOLOGY 



it, because its image falls on cones alone, while it becomes visible 

 if the eye be directed slightly from it. The rods seem incapable 

 of giving rise to colour sensation, and when the solar spectrum 

 is looked "at in a very dim light it appears as a greyish band of 

 illumination with the red end wanting, because the slow red 

 vibrations fail to stimulate the rods. It is because the blue 

 end of the spectrum is the more active in faint illumination 

 that the illusion of a moonlight scene may be got by looking 

 through a blue glass, while looking through a yellow glass 

 gives the idea of sunlight and brilliant illumination. (Practical 

 Physiology.) 



(4) Modes of Stimulation. The rods and cones are generally 

 stimulated by the ethereal light vibration, but they may be 

 stimulated by mechanical violence or by sudden changes in 

 an electric current. But, however stimulated, the kind of 

 sensation is always of the same kind a visual sensation (see 

 p. 118). 



(5) Of the nature of the changes in the retina when stimu- 

 lated we know little. But we know that 



1st. Under the influence of light the cells of the tapetum 

 nigrum expand forward between the rods and cones. 



2nd. A purple pigment which exists in the outer segment 

 of the rods is bleached. Even although there is no purple 

 in the cones, which alone occupy the sensitive central spot 

 of the eye, this change in colour suggests that a chemical de- 

 composition accompanies stimulation. 



3rd. Electrical changes occur. 



(6) Fatigue. If a bright light be looked at for some time, 

 the part of the retina acted upon is temporarily blinded, and 

 hence when the eye is taken off the bright light a dark spot is 

 seen. This is sometimes called a negative after image. When 

 coloured lights are used the phenomena of complemental colours 

 are produced (p. 135). (Practical Physiology.) 



Sometimes the stimulation of the retina or of the brain 

 neurons connected with it may last after the withdrawal of 

 the stimulus, when a continuance of the sensation a positive 

 after image is seen. This may be observed if, on opening the 

 eyes in the morning, a well-illuminated window is looked at and 

 the eyes closed. A persisting image of the window may be 

 seen. 



