SPECIAL SENSES. 



taste of sapid substances, odors, etc., as the result, to a great degree, of education. In 

 the same way, probably, we acquire the power of noting the position of objects m vision ; 

 but even this supposition is not necessary to explain the phenomenon of direct vision by 

 means of inverted images. The following paragraph, quoted from Giraud-Teulon, is a 

 simple expression of facts and shows the absurdity of the elaborate theoretical explana- 

 tions made by many of the earlier writers : 



" If the objects seen mark their image upon the retina, each one in a proper second- 

 ary axis ; if, on the other hand, the retina appreciates these, independently of ourselves, 

 in these' same secondary axes, which all cross at the same point, it is evident that an 

 exact or erect sensation, as well as the object which produces it, should necessarily corre- 

 spond to an inverted or reversed image. But it is neither habit, education, nor informa- 

 tion derived from the sense of touch, that enables us, as it is said, to see objects erect 

 by means of reversed images. The retina sees or localizes objects where they are ; that is 

 what we call ' erect.' If the picture be reversed, it is a mere matter of geometry." 



In discussing the same question, Helmholtz says that " our natural consciousness is 

 completely ignorant even of the existence of the retina and of the formation of images : 

 how should it know any thing of the position of images formed upon it ? " 



J3inocular Vision. 



We have thus far considered the mechanism of the eye and its action as an optical instru- 

 ment, in simple, or monocular vision. It is evident, however, that we habitually use both 

 eyes, and that their axes are practically parallel in looking at distant objects and are con- 

 verged when objects are approached to the nearest point at which we have distinct vision. 

 In fact, an image is formed simultaneously upon the retina of each eye, but it is neverthe- 

 less appreciated as a unit. If the axis of one eye be slightly deviated by pressure upon 

 the globe, so that the images are not formed upon corresponding points upon the retina 

 of each eye, our vision is more or less indistinct and is double. In strabismus, when this 

 condition is recenf, temporary, or periodical, as in recent cases of paralysis of the exter- 

 nal rectus muscle, when both eyes are normal, there is double vision. "When the strabis- 

 mus is permanent and has existed for a long time, double vision may not be observed, 

 unless the subject direct the attention strongly to this point. As it is usual, in such 

 cases, for one eye to be much superior to the other in acuteness of vision, an object is 

 fixed with the better eye, and its image is formed upon the fovea. The image formed 

 upon the retina of the other eye is indistinct, and in many instances it is habitually disre- 

 garded ; so that, practically, the subject uses but one eye, and presents the errors of 

 appreciation which attend monocular vision, such as a want of accurate estimation of 

 the solidity and distance of objects. It is stated as the rule that, when strabismus of 

 long standing is remedied, as far as the axes of the eyes are concerned, by an operation, 

 binocular vision is not restored ; but the experiments necessary to the accurate determi- 

 nation of this point are exceedingly delicate and must be made with great care. This is 

 explained upon the supposition that the functional power of the retina of the affected 

 eye has been gradually and irrecoverably lost from disuse. In normal binocular vision, 

 the images are formed upon the fovea centralis of each eye ; that is, upon corresponding 

 points, which are, for each eye, the centres of distinct vision. 



It is hardly necessary to speculate with regard to the reason why two images, one 

 upon each retina, convey the impression of a single object. We appreciate a sound with 

 both ears; the impression of a single object is received by the sensory nerves of two. or 

 more fingers; the olfactory nerves upon the two sides are simultaneously concerne 

 olfaction ; and, in the same way, when we look at a single object with both eyes, the 

 brain appreciates a single image. We shall see, however, that the concurrence of both 

 eyes is necessary to the exact appreciation of distance and form ; and, when the two iir 

 are formed upon corresponding points, the brain receives a correct impression of a . 



