SIGHT. 503 



as seen by the two eyes, when placed exactly in front of the ob- 

 server at the distance of eighteen inches or two feet, the right eye 

 will see the object partly on one side, and the left eye partly on the 

 other. And by the union or combination of these two images by 

 the visual organs, the impression of solidity is produced. 



By the employment of double pictures, so drawn as to represent 

 the appearances presented to the two eyes by the same object, and 

 so arranged that each shall be seen only by the corresponding eye, 

 a deceptive resemblance may be produced to the actual appearance 

 of solid objects. This is accomplished in the contrivance known 

 as the Stereoscope. Thus, if two pictures similar to those in Figs. 

 163 and 164 be so placed that one shall be seen only with the right 

 eye and the other only with the left, the combination of the two 

 figures will take place as if they came from the real object, and 

 all the natural projections will come out in relief. 



But this effect is produced only in the case of objects situated 

 within a moderately short distance. For very remote objects, we 

 lose the impression of solidity, since the difference in the images on 

 the two eyes becomes so slight as to be inappreciable, and we see 

 only a plane expanse of surface, with sharp outlines and various 

 shades of color, but no actual projections or depressions. 



The sensibility of the retina is such that it cannot distinguish 

 luminous points which are received upon its surface at a very 

 minute distance from each other. In this particular, the sensibility 

 of the retina resembles that of the skin, since we have already 

 found that the integument cannot distinguish the impressions 

 made by the points of two needles placed a very short distance 

 apart. The delicacy of this discriminating power, in the retina, is 

 immeasurably superior to that of the skin ; and yet it has its 

 limits, even in the nervous expansion of the eye. For if we look 

 at an object which is excessively minute, or which is so remote 

 that its apparent size is very much diminished, we lose the power 

 of distinguishing its different parts, and can no longer perceive 

 its real outline. This is a very different condition from that in 

 which the confusion of vision arises from defect of focusing in the 

 eye, as, for example, in long or short-sightedness, or where the 

 object is placed too near the eye or too much on one side. For 

 when the difficulty depends simply on its minute size or its remote- 

 ness, the rays coming from the top of the object and those coming 

 from the bottom, are all brought to their proper focus at distinct 

 points on the retina only these points are too near each other for the 



