456 SPECIAL PHYSIOLOGY. 



to be single. So, likewise, in the use of the doubly refracting Iceland 

 spar, and of multipljing-glasses or reflectors, the pencils of luminous 

 rays from one object, are so refracted or reflected, as to form double, 

 or multiple images, which are thrown on to different parts of the same 

 retina, and accordingly are seen as multiple images, though known, on 

 other grounds, to proceed from one object. 



Each retina regards as single, an image formed on any one definite 

 point of its surface ; but single objects necessarily form an image in 

 both eyes, and hence, for the useful and undeceptive application of 

 vision, the mind must be able to combine the impressions made by 

 these two images, so as not to be deceived into a belief that they pro- 

 ceed from two objects, instead of from one. 



When both eyes are thus directed to any given object, lines pro- 

 longed from their optic axes, meet at that object, and the two retinal 

 images produced by it, viz., that formed in the right eye and that 

 formed in the left, fall exactly on the centres of the two retinae. The 

 images of all surrounding objects are received on surrounding por- 

 tions of the two retinal surfaces. Those portions, like the central 

 points themselves, are said to correspond, though of course, from the 

 complete inversion of the pictures, they are on opposite sides of the 

 centres of the retinae. The two pictures thus received, are not seen 

 separately, and referred to two sets of objects, but are combined by the 

 mind, and referred to a single set of objects. In other words, single 

 vision with two eyes, results. Sometimes, however, impressions made 

 on the two retinae are not combined, but are separately distinguished 

 by the mind, producing the phenomenon called double vision. It is 

 probable that in the infant, double vision, and, therefore, indistinct 

 vision, is, at first, the rule, but that, by degrees, the eyes are brought 

 to converge suitably on external objects, and thus single vision occurs. 

 By habit and education, this power is, at length, so confirmed, that we 

 are no longer conscious of perceiving two images, but only experience 

 a singleness of perception. When double vision occurs, it is found 

 that the two eyes are not turned directly towards a given object, so 

 that lines prolonged from their axes no longer meet in that object, and 

 the images of it, formed in the two eyes, are no longer received on 

 corresponding points of the retinae. 



These corresponding or identical points of the retinae, are, by some, 

 regarded as the result of use or habit alone ; but, on the other hand, 

 their existence, as fundamentally or primitively identical spots, is 

 thought to be proved by the following experiments. If pressure be 

 made, in the dark, or when the eyelids are closed, upon the upper por- 

 tion of both eyeballs, a single luminous circle, named a phozphene 

 (Serre), is perceived in the centre of the field of vision below. If the 

 lower part of both eyeballs be pressed upon, a single luminous circle 

 is perceived in the centre of the field of vision above. Hence the 

 upper portions and the lower portions of the two retinae, are regarded, 

 respectively, as identical. Pressure upon the inner side of one eyeball 

 and the outer side of the other, produces a single circle of light, in a 

 direction opposite to that of the pressure ; hence the inner side of one 

 retina is said to correspond with the outer side of the other. But if 



