VISUAL JUDGMENTS 445 



retina are called right, left, upper, lower, quite irrespec- 

 tively of their real positions, of which the mind has, and 

 can have, no cognizance. 



6. Single Objects give rise to Single Images.— TF/ien 

 on external body is ascertained by touch to be simple, it 

 forms but one image on the retina of a single eye ; and when 

 two or more images fall on the retina of a single eye, they 

 ordinarily proceed from a corresponding number of bodies 

 which are distinct to the touch. 



Conversely, the sensation of tivo or more images is judged 

 by the mind to proceed from two or more objects. 



If two pin-holes be made in a piece of cardboard at a 

 distance less than the diameter of the pupil, and a small 

 object like the head of a pin be held pretty close to the 

 eye, and viewed through these holes, two images of the 

 head of the pin will be seen. The reason of this is, that 

 the rays of light from the head of the pin are split by the 

 card into two minute pencils, which pass into the eye on 

 either side of its centre, and, on account of the nearness 

 of the pin to the eye, meet the retina before they can be 

 united again and brought to one focus. Hence they fall 

 on different parts of the retina, and each pencil of rays 

 being very small, makes a tolerably distinct image of its 

 own of the pin's head on the retina. Each of these 

 images is now referred outward (p. 444) and two pins are 

 apparently seen instead of one. A like explanation 

 applies to multiplying glasses and doubly refracting crystals, 

 both of which, in their own ways, split the pencils of 

 light proceeding from a single object into two or more 

 separate bundles. These give rise to as many images, 

 each of which is referred by the mind to a distinct 

 external object. 



7. The Judgment of Distance and Size by the 

 Brightness and Size of Visual Images.— Ccrtoi?i visual 

 phenomma ordinarily accompany those products of tactile 

 sensation to which we give the name of size, distance, and 

 form. Thus, other things being alike, the space of the retina 



