210 PHYSIOLOGY. 



the eyes placed on opposite sides of the head, possess in a 

 more remarkable degree the faculty of thus attending to one 

 thing at a time. 



30. It follows then, that the eye itself does not see ; it is 

 only an instrument employed by the brain, or rather the 

 mind, whose servant the brain is. The optic nerve is the 

 channel by which the mind peruses the hand writing of na- 

 ture on the retina, and through which it transfers to that 

 material tablet its decisions and its creations. There is then 

 a portion, or rather an organ of the brain, where vision or 

 the mind's eye is seated ; and this portion is sometimes im- 

 perfectly constituted or organized, as Gall has proved to be 

 the case, in those who cannot distinguish colours. That 

 such is a true statement of facts, we learn from observing that 

 disease of a given portion of brain produces blindness, whilst 

 the eye remains perfectly healthy. Either this was the case 

 with Milton, or he was afflicted with amaurosis, or palsy of 

 the retina, for he says, 



" These eyes, though clear 

 To outward view, of blemish, or of spot, 

 Bereft of light, their seeing have forgot! 

 Nor to their idle orbs doth sight appear 

 Of sun, or moon, or star throughout the year !" 



We also have the power of internal vision when asleep ; and 

 those who have lost their eyes perceive spectral illusions, and 

 other similar phenomena, and they can alio conjure up the 

 figures and forms of various objects familiar to them before 

 they lost their vision. ^gg 



31. {From the principles now laid down we can understand 

 why rivers appear shallower than they are, and why a stick 

 placed in the water appears crooked. In spearing of fish, 

 or shooting them in the water, every sportsman knows that 

 he must make suitable allowance for this refraction, for the fish 

 is always nearer to him than it appears. Birds that dive for 

 fish seem to understand that there is but small chance of 

 success if they dive obliquely into the water, so like skilful 



