SENSE OF SIGHT. 



583 



Form. Plane surfaces can be seen with either eye alone, but 

 solid bodies require the combined use of bofch eyes, or binocular 

 vision, in order that their solidity may be appreciated. If a solid 

 object is looked at with the left eye, while the right eye is closed, 

 and then with the right eye while the left is closed, it will be ob- 

 served that with the left eye more of the left side of the object is 

 seen than with the right eye, and with the right eye more of the 

 right side of the object than with the left eye. The two images 

 produce the effect in the brain of a single solid body. This 

 principle is made use of in the stereoscope (Fig. 361). The 

 picture which is seen with this instrument is double, each having 

 been taken with a separate lens, 

 so that when their images are 

 thrown on the retina the effect is 

 as if both eyes were looking at 

 the scene represented in the pho- 

 tographs. 



Identical Points. It would 

 seem a priori that each retinal 

 image would produce its own ef- 

 fect upon the brain, and that, in- 

 stead of seeing a single object, it 

 would appear double. The theory 

 of identical or corresponding points 

 has been advanced to explain what 

 actually takes place. If one retina 

 is in imagination placed upon the 

 other, the foveae centrales super- 

 imposed the one upon the other, 

 all the other points of the retinae 

 similarly 'superimposed are iden- The prisms refract the rays coming 



tioil or oorresnondino* noints and from the points c ' r of the P ictures ab 

 ing po nis, anc and a ^ so that they appear to come 



images formed upon such points from a single point, q. Similarly the 



will in the brain produce the ef- points a and a appear to be situated at 



J ... , . . r , T7 , , , /, and the points 6 and ft at <<>. 

 feet or single vision. When the 



images of an object are not formed upon identical points, double 

 vision or diplopia results. If, therefore, while looking at an object 

 we press with the finger upon the outer side of one eye so as to 

 turn it a little inward, we see the object double. 



Size. The size of objects is determined by two factors : The 

 size of the visual angle which they subtend, and their distance 

 from the observer. Helmholtz has demonstrated that an object 

 which subtends a visual angle of less than 50" cannot be seen by 

 the unaided emmetropic eye. This corresponds to an image on the 

 retina of 3.65 p.. The diameter of each cone in the macula 

 lutea is about 3 p. Kolliker gives it as 4 p. to 5 p. In other 

 words, if an object does not make an image on the retina large 



FIG. 361. Brewster's stereoscope: 



