120 SPECIAL SENSES. 



the eyes, and the first appears double. It is evident here, 

 that when the axes of the eyes bear upon one of these objects, 

 the images of the other must be formed a certain distance 

 from the corresponding retinal points. 



The Horopter. The above-mentioned experiment enables 

 us to understand the situation of the horopter. If we fix 

 both eyes upon any object directly in front and keep them in 

 this position, a similar object moved to one side or the other, 

 within a certain area, may be seen without any change in the 

 direction of the axis of vision ; but the distance from the eye 

 at which we have single vision of this second object is fixed, 

 and, at any other distance, the object appears double. The ex- 

 planation of this is, that, at a certain distance from the eye, the 

 images are formed upon corresponding points in the retina ; 

 but, at a shorter or longer distance, this cannot occur. This 

 illustrates the fact that there are corresponding points through- 

 out the sensitive layer of the retina, as well as in the f ovea 

 centralis. By these experiments, the following facts have 

 been ascertained : With both eyes fixed upon an object, an- 

 other object moved to one side or the other can be distinctly 

 seen only when it is carried in a certain curved line. On 

 either side of this line, the object appears double. This line, 

 or area, for the line may have any direction, is called the 

 horopter. It was supposed at one time to be a regular curve, 

 a portion of a circle drawn through the fixed point and the 

 points of intersection of the rays of light in each eye. Though 

 it has been ascertained that the line varies somewhat from a 

 regular curve, and also varies in different meridians, this is 

 due to differences in refraction, etc., and the principle is not 

 altered. 



It is undoubtedly true that education and habit have a 

 great deal to do with the correction of visual impressions 

 and the just appreciation of the size, form, and distance of 

 objects. If we may credit the account of the remarkable 



