Intelligence and Miscellaneous Articles. 325 



The field of vision of each eye subtends an angle of 200°, but it 

 is reduced on one side in the horizontal direction according to the 

 projection of the nose. It is also reduced in the vertical line accord- 

 ing to the projection of the frontal bone. 



With the two eyes, the shadow of the nose, being in an opposite 

 position for each of them when the two eyes are open, does not 

 impede the full range of vision, which embraces the whole extent 

 of 200°. 



To explain how the optical combination of the eye can form on 

 the retina an image of 200°, we have to assume that the rays of 

 light in passing through the cornea and the crystalline lens are more 

 and more refracted in proportion to the angle at which they strike 

 the spherical surface of the cornea. By this refraction the rays 

 which enter the eyes at an angle of 90° are bent 10°, and appear to 

 come from an angle of 80°. 



This phenomenon, which I have not seen noticed in any treatise 

 on optics, produces a very curious illusion. When we are lighted 

 by the sun, the moon, or any other light, if we endeavour to place 

 ourselves in such a position that the front of the body should be 

 on a line with its own shadow and with the light which produces 

 it, if we look straight before us we see unexpectedly with one eye 

 the light and with the other the shadow of our body ; and although 

 they must form an angle of 1 80°, they appear by the refraction to 

 form one of only 160°. In fact the light and the shadow do not 

 seem to be connected at all ; instead of being in a line they appear 

 bent to an angle. The shadow seems to come not from the light 

 we actually see, but from one which ought to be a little behind 

 us; and as to the light, it seems that it should only be capable 

 of forming the shadow of our body enough behind, us to be invi- 

 sible. In other words, the ray of light and the shadow which 

 ought to coincide and form only one line, are bent by the refractive 

 power of the eye, like the stick, one half of which being immersed 

 in water, appears crooked or bent into an angle at the point of 

 immersion. 



This curious phenomenon evidently proves that the process of 

 vision is affected by the common law of refraction, and that the 

 only objects which appear in their true position are those the image 

 of which, entering the eye in the direction of the optic axis, undergo 

 no refraction. 



So that mathematically we see in their right place only the objects 

 which reflect their light on the centre of the retina, and all the other 

 objects are more and more refracted as they enter the eye in a more 

 and more oblique direction. Therefore the panoramic picture which 

 is represented on the retina is correct only in the centre of this 

 organ, and all the other parts of the picture are seen not exactly 

 where they are. But this is no imperfection in the general effect, as 

 it does not affect in the least the perception of the picture, because, 

 as we have only a distinct vision for the parts which are represented 

 on the centre of the retina, when we want to see the whole pano- 

 rama we have to direct gradually the optic axis upon the various 



