638 THE SENSES. 



This fact is fully demonstrated by the phenomena of angular reflec- 

 tion and refraction. If a candle be held behind the back, in such a posi- 

 tion as to be reflected in a mirror placed at the front, the light presents 

 itself to the eye as if it were really in front, because it is from this 

 direction that the luminous rays finally come. If we observe the reflec- 

 tion of objects in a mirror held horizontally, or in a smooth sheet of 

 water, the objects seem to be placed below the reflecting surface, although 

 they are really above it ; since the rays which make their impression 

 upon the eye actually come from below. A stick or pebble, seen ob- 

 liquely at the bottom of a transparent pool, appears nearer the surface 

 than it really? is, because the rays which reach the visual organ have 

 been bent from their course, in passing from the water into the atmos- 

 phere, and have consequently assumed a more oblique direction. 



Erect Vision, with Inverted Retinal Image. Since it is the direction 

 of the visual rays, rather than the point of their impact upon the retina, 

 which determines the apparent relative position of luminous objects, 

 such objects appear erect although their images upon the retina are 

 inverted. The retinal image is not the form which is seen by the eye 

 itself, but is only a phenomenon visible to the inspection of another 

 eye. It is an appearance which is incidental to the mode of refraction 

 of the visual rays ; and its position is quite a distinct matter from that 

 of the luminous impressions perceived by the retina. Its relation to 

 the picture really presented to the sensitive membrane, is like that of 

 the reversed engraving upon a wood-cut to the printed impression of 

 the same design ; or like that of the elevations and depressions of a 

 mould to the depressions and elevations of the cast taken from it. In 

 the field of sight, therefore, for each eye, every object appears above or 

 below, to the right or left, according to the position which it really 

 occupies in regard to the centre of the field and the line of direct vision. 



Point of Fixation, in Vision with Two Eyes. For each eye, distinct 

 perception is possible, as shown above (p. 628), only for objects situated 

 in a single range, which is known as the " line of direct vision." Since 

 the eyes are placed in their orbits at a lateral distance from each other 

 of about six centimetres, when they are both directed at the same object, 

 within a moderate distance, their lines of direct vision have a sensible 

 convergence, and, of course, cross each other only at a single point. 

 At this point of intersection of the two lines of direct vision, an object 

 may be seen distinctly by both eyes at the same time. But at every 

 other point, it must appear indistinct to one of them ; because if it be 

 in the line of direct vision for the right eye it will be out of that line 

 for the left, and vice versa. There is, accordingly, only a certain dis- 

 tance, directly in front, at which an object can be distinctly seen sim- 

 ultaneously by both eyes ; namely, that at which the two lines of direct 

 vision cross each other. This point is called the point of fixation, for 

 the two eyes. In fixing any object, for binocular vision, the accommo- 

 dation in each eye is at the same time adjusted for the required distance ; 



