116 SPECIAL SENSES. 



apparatus, it will be sufficient to note the action of both eyes, 

 as contrasted with the action of one, in normal vision, with- 

 out discussing fully the multitude of curious observations 

 made with the stereoscope ; and we can readily comprehend 

 the action of muscles by which the axis of vision is directed 

 toward different objects, without entering into a discussion 

 .of abstruse mathematical calculations with regard to the 

 exact centre of rotation, the law of torsions, and other points 

 connected with physiological optics. These are questions, 

 however, of great interest to ophthalmologists, and are fully 

 discussed in elaborate special treatises. 



We shall allude briefly, in this connection, to a question 

 which has long engaged the attention of physiologists, and one 

 which, we cannot but think, has been made the subject of 

 much unprofitable speculation. It is a matter of positive 

 demonstration that the images of objects seen are inverted 

 as they appear upon the retina. Why is it, however, that 

 objects are appreciated as erect, when their images are thus 

 inverted ? With a knowledge of the fact that the apprecia- 

 tion of impressions made upon the nerves of special sense is 

 capable of education and is corrected by experience, it seems 

 hardly necessary to enter into an elaborate discussion of this 

 point. We appreciate with accuracy the density of objects, 

 the direction of sounds, differences in musical tones, the 

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

 degree, of education. In the same way, probably, we ac- 

 quire the power of noting the position of objects in 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 explanations made by many of the ear- 

 lier writers : 



" If the objects seen mark their image upon the retina, 

 each one in a proper secondary axis ; if, on the other hand, 

 the retina appreciates these, independently of ourselves^ in 



