654 THE SENSE OF SIGHT. 



leads us to a consideration of the several other properties 

 of the retina, and of the co-operation of the mind in the 

 several other parts of the act of vision. To these belong 

 not merely the act of sensation itself, and the perception 

 of the changes produced in the retina, as light and colours, 

 but also the conversion of the mere images depicted in the 

 retina into ideas of an extended field of vision, of proxi- 

 mity and distance, of the form and size of objects, of the 

 reciprocal influence of different parts of the retina upon 

 ach other, the simultaneous action of the two eyes, and 

 some other phenomena. 



To speak first of the ideal size of the field of vision : The 

 actual size of the field of vision depends on the extent of 

 the retina, for only so many images can be seen at any one 

 time as can occupy the retina, at the same time ; and thus 

 considered, the retina, of which the affections are perceived 

 by the mind, is itself the .field of vision. But to the mind 

 of the individual the size of the field of vision has no 

 determinate limits; sometimes it appears very small, at 

 another time very large ; for the mind has the power of 

 projecting the images on the retina towards the exterior. 

 Hence the mental field of vision is very small when the 

 sphere of the action of the mind is limited to impediments 

 near the eye : on the contrary, it is very extensive when 

 the projection of the images on the retina towards the 

 exterior, by the influence of the mind, is not impeded. It 

 is very small when we look into a hollow body of small 

 capacity held before the eyes; large when we look out 

 upon the landscape through a small opening ; more exten- 

 iSive when we look at the landscape through a window ; 

 and most so when our view is not confined by any near 

 object. In all these cases the idea which we receive of 

 the size of the field of vision is very different, although its 

 absolute size is in all the same, being dependent on the 

 extent of the retina. Hence it follows, that the mind is 

 constantly co-operating in the acts of vision, so that at last 



