SINGLE VISION WITH TWO EYES. 455 



eye, for the angle it subtends must, of course, be larger, the closer its 

 proximity to the eye. The apparent magnitudes of a small object, 

 close to the eye, and of a large object at a distance, such as a pin and 

 a man, are identical, if they subtend equal angles. The degree of 

 movement of the eyeballs, required to pass from one end of the object 

 to the other, is also a further means of determining apparent magni- 

 tudes. The sense of magnitude is more exact in regard to horizontal 

 than to perpendicular lines. It is said to be possible to distinguish 

 between two lines of different lengths, even after certain intervals 

 of time, for example, to the extent of a difference of J th after the 

 lapse of 3 seconds, and of -f'j-th after 70 seconds. The accordance of 

 the senses of sight and touch, as regards the information which they 

 respectively afford, concerning the size of objects, seems to be the 

 result of experience and comparison ; for a person born blind, who 

 gained sight by an operation, has been said to state that objects known 

 to him by touch, appeared larger than he expected. 



Our knowledge of the real magnitude, or absolute size, of objects, 

 is only arrived at indirectly, or by means of experience or inference, 

 by comparing them with objects, with the dimensions of which we are 

 already familiar, and by taking into account their respective distances. 



In the same manner, our visual sensations inform us only of the 

 apparent or superficial shape and color of objects, of their apparent 

 direction or position in the field of vision, and of their apparent mo- 

 tion in the same. All our conclusions as to their real form, color, po- 

 sition, and motion, are arrived at by observation and comparison of 

 these appearances. For the determination of the apparent qualities 

 of any object, and also of its real size and color, one eye alone suffices: 

 but for the purpose of ascertaining, by means of the sight alone, its 

 real form, real position, and real motion, the conjoined use of both 

 eyes affords material assistance. In this constant mode of employing 

 the two eyes, their distance from each other, named the inter-ocular 

 distance, is of the highest optical importance. 



We derive our notions of the solidity, roundness, or relief of ob- 

 jects, from the combined use of the two eyes ; for when one eye alone 

 is employed, we can only see plane figures having two dimensions, viz., 

 length and breadth. For the perception of solid forms of three di- 

 mensions, viz., length, breadth, and thickness, within a moderate dis- 

 tance, the optic axis of the two eyes are made to converge, so that 

 straight lines prolonged from them would meet in the object. 



As a rule, an external object forms but a single image in one eye, 

 and the mind, perceiving such single image, refers it to a single object. 

 But there are conditions in which one eye may receive two or more 

 identical images, from, and of, one external object, and then, unless 

 the mind be otherwise informed of the illusion, such images are referred 

 by it to as many distinct, though exactly similar objects. For ex- 

 ample, if we look with one eye at a pin through two minute holes in a 

 card, the distance between which is less than the diameter of the pupil, 

 the same retina receives, on different parts of its surface, two separate 

 images of the pin, which, accordingly, being outwardly projected along 

 the proper lines of direction, are seen double, though we know the pin 



