THE EYE. 



similar to that wliicli would be produced by the mixture of red 

 with white. t 



This, however, is not the case. If the experiment be performed 

 as here described, the eye will, on the removal of the red wafer, 

 perceive, not a reddish, but a greenish-blue disk. 



In like manner, if the wafer, instead of being red, were of a 

 bright greenish -blue, when removed the impression on the eye 

 would be that of a reddish disk. 



These and like phenomena are explained as follows : 



When the eye is directed with an intensity of gaze for some 

 time at the red surface, that part of the retina upon which the 

 image of the red wafer is produced becomes fatigued with the 

 action of the red light, and loses to some extent its sensibility to 

 that light, exactly as the ear is deafened for a moment by an 

 overpowering sound. When the red wafer is removed, the white 

 disk beneath it transmits to the eye the white light, which is 

 composed of all the colours of the spectrum. But the eye, from 

 the previous action of the red light, is comparatively insensible 

 to those tints which form the red end of the spectrum, such as red 

 and orange, but comparatively sensible to the blues and greens, 

 which occupy the other end. It is therefore that the eye 

 perceives the white disk as if it were a greenish-blue, and con- 

 tinues to perceive it until the retina recovers its sensibility for 

 red light. 



71. A difficulty has been presented in the explanation of the 

 functions of the eye to which, as it appears to me, undue weight 

 has been given. It has been already explained, that the images 

 of external objects which are depicted on the retina are inverted ; 

 and it has accordingly been asked why visible objects do not 

 appear upside down. The answer to this appears to be extremely 

 simple. Inversion is a relative term, which it is impossible to 

 explain or even to conceive without reference to something which 

 is not inverted. If we say that any object is inverted, the phrase 

 ceases to have meaning unless some other object or objects are 

 implied which are erect. If all objects whatever hold the same 

 relative position, none can be properly said to be inverted ; as the 

 world turns upon its axis once in twenty-four hours, it is certain 

 that the position which all objects hold at any moment is inverted 

 with respect to that which they held twelve hours before, and to 

 that which they will hold twelve hours later ; but the objects as 

 they are contemplated are always erect. In fine, since all the 

 images produced upon the retina hold with relation to each other 

 the same position, none are inverted with respect to others ; and 

 as such images alone can be the objects of vision, no one object of 

 vision can be inverted with respect to any other object of vision ; 

 78 



