46 BEING AND FACULTIES OF MAN. 



extent portrayed on the retina of the bird could not be 

 distinctly traced by us without the aid of the most power- 

 ful microscope, though all distinctly seen by the bird 

 itself without any such aid ; and the same image on the 

 retina of the ox, though many thousand times larger than 

 that presented to the consciousness of the bird, could not 

 be traced by us in many of its most prominent details with- 

 out having again recourse to optical science. On the retina 

 of the eye of the bird, a railway train passing over twenty 

 miles of distance in the remote horizon of such a land- 

 scape will not describe a line longer than the breadth of a 

 hair, and yet it will be as distinctly and as accurately seen by 

 a bird as by a man, because the relative sizes and distances 

 of all the objects seen will be maintained, though their 

 true dimensions are not given by the eye either of the 

 man or the bird. But this question will be more fully 

 understood by referring also to the subject of erect vision 

 as it is called, or why, notwithstanding the fact that all 



images are inverted on the retina, we nevertheless see 

 them in their proper and erect position. Thus an arrow 

 placed with the point upwards before the eye, ABC, 

 would, as shown in the above figure, be inverted on the 

 retina at c b a, or portrayed with the point downward, 

 because the rays m n o, and indeed all the rays proceed- 

 ing from the arrow between the extremities of it, A C, and 

 proceeding to the convex surface of the eye, x y, would 



