, Univ. of Calitbrnia.* 



IV. The mode of representing the position of double images. 



It is well known that if two objects, as a finger of each hand, 

 be placed one beyond the other in the median line of sight, 

 when the eyes are' fixed upon the nearer object the farther ob- 

 ject is seen double, the images being homonymous, i. e. on the 

 same side as the eyes to which they belong : and when the eyes 

 are fixed upon the farther object the nearer object is seen 

 doubled, the images being in this case heteronymous, i. e, opposite 

 the eyes to which they belong. These familiar facts are usually 

 represented graphically as follows : Let R and L, figs. 1 and 2, 

 represent the right and left eye, and A and B the two objects. 

 Now when the eyes are directed upon A, tig. 1, then the light 

 from B will impress the temporal halves of both retinae and B 

 will be seen by the right eye at h and by the left eye at h' 

 (heteronymous) ; but if the eyes be directed upon B fig. 2, A 

 will impress the nasal sides of the two retinae, and be seen as 

 homonymous images at a and a'. (In all cases in this paper ob- 

 jects seen single are represented by capitals, right eye images by 

 plain italics, and left eye images by dashed italics). It will be 

 observed that in both cases the doubled images are referred to 

 a plane passing through the point of sight at right angles to 

 the visual plane. For convenience I will call this the plane of 

 sight. Now every one who has ever tried the experiment knows 

 that the double images are not thus referred in natural vision, 

 but on the contrarv are seen at their real distance, though not 

 in their real position. The figures therefore though they truly 

 represent the parallactic position of the double images do not 

 represent truly their apparent distance. If on the other hand 

 we attempt in our figures to refer the images to their proper 

 distances, observing the law of direction, then they unite and 

 form one; which °is equally incorrect. It is evident there- 

 fore that these figures cannot represent truly the visual results. 



The falseness of this mode of representation becomes much 

 more conspicuous, if instead of two points or small objects, we 

 substitute a line or rod. In this case the absurdity of project- 

 ing the images on the plane of sight is so evident that it is never 

 attempted. The universal mode of representing the visual result 

 of a rod placed in the median line of sight is shown m the ac- 

 companviuff figures. Fig. 3 represents the actual position ot 

 the rod AB in the median line of sight, fig. 4, the visual result 

 when the eyes are directed upon A, and fig. 5 the visual result 

 * For the preceding articles on this subject, see II, ilviii, 68, 153. 



Am. Joub. Sci.— Third Series, Vol. I, No. 1.- Jan., 1871. 



