472 THE POPULAR SCIENCE MONTHLY. 



forms the frame of the field of view, and what is within this frame 

 is itself the field of view. 



This space, immovable as regards the head, becomes the arena to 

 which both those excitations produced by the senses, and those be- 

 longing to subjective sight, are transposed; and this transposition al- 

 ways takes place in the same direction as that in which a regular act 

 of sight would in any case lie. Although long experience has taught 

 us it is a delusion, still we always place the image projected by a mir- 

 ror behind it, because the reflected light falls into our eye just as if 

 the object were behind it. But the sparks produced by pressing the 

 eye from the side of the temples, we seem to see in the opposite side 

 of the field of view ; but we do so because, in the normal act of sight, 

 the retina is irritated (from the side of the temples) by light falling 

 from the opposite side. By the action of projection, the reversed 

 image naturally regains its upright position in the field of view. 



A general irritation of the retina unaccompanied by a perception 

 of objects will give us a light field of view, and, on the other hand, a 

 perfect repose of those parts will give us a dark field of view. The 

 former represents the sense of the repose of a mechanism endowed 

 with the power of action ; the latter corresponds to the absence of all 

 mechanism whatever. The feeling of darkness, therefore, results 

 merely from the expansion of your field of view as opposed to your 

 retina, if I may so say ; while behind your back you have the feeling 

 neither of light nor of darkness ; you simply miss all sensation of light. 



Touching the size of the images on the retina, as compared with 

 the objects themselves, I need merely remind you of the rules of per- 

 spective. The images on the retina stand in reversed proportion to 

 the distance of the objects. The image of a pencil, held a foot from 

 your eye, covers the trunk of the tree before your window ; that of a 

 pea, at a like distance, covers the moon in the sky. If, notwithstand- 

 ing, we think the moon bigger than the pea, and the tree than the 

 pencil, the reason is that, apart from our being well acquainted with 

 the tree, our judgment is a combination of the size of the image on the 

 retina and the distance of the object. Now, as consciousness is for 

 the most part founded on experience, so just and correct perspective 

 sight is in the main something we acquire. A child will assuredly not 

 appreciate the difference between the pencil and the trunk of the tree 

 in the same degree as an individual who by experience has learned to 

 know the value of his impressions. What the child first knows of the 

 moon is, that he cannot reach it with his hand ; " But," as I once heard 

 a child say, "mother can reach it down." Other inferences have 

 helped him to this conclusion. We are so accustomed to merely play 

 with children, that we are easily blinded to the full seriousness of such 

 requests. 



We cannot break off these reflections on the image of the retina 

 without making mention of a remarkable spot in the background of 



