244 ANIMAL PHYSIOLOGY. 



superior and inferior recti, acting by themselves, will be no 

 longer capable of rotating them directly upwards and down- 

 wards, seeing that the axis of vision and the direction of 

 these muscles are no longer in the same vertical plane; and 

 the oblique muscles are so disposed that, by acting in concert 

 with the superior and inferior recti, they are capable of com- 

 pensating for the deviation of the direction of these muscles 

 from the axis of vision, and so maintain the vertical diameters 

 of the eyeballs parallel to one another, which we shall im- 

 mediately see to be a condition necessary for perfect vision. 



179. When we look at an object with both eyes directed full 

 upon it, we see it as a single object, notwithstanding that two 

 images of it are received, one on each retina. But by arti- 

 ficial expedients these two images may be made to give the 

 appearance of two objects; that is to say, double vision may 

 be produced. By pressing a finger gently on the side of one 

 eyeball, so as to derange the position of its axis of vision, a 

 second picture of each ol^ject in the landscape may be made 

 to appear, either above, below, or to one side of the more 

 distinct picture presented to the other eye, according to the 

 direction of pressure. Or, if a finger be held up exactly in 

 front of a more distant object, and the eyes be directed to 

 the finger, while the attention takes cognisance of the object 

 beyond, that object will be seen double. On the other hand, 

 in looking through a stereoscope, two pictures have the 

 appearance of one. All these phenomena depend on one 

 law, which may be expressed thus : that rays which fall on 

 points in the outer half of one retina, are referred by the 

 mind to the same direction as those which fall on similarly 

 situated points in the inner half of the other retina. Such 

 points are therefore said to be physiologically corresponding 

 or identical. When one eye is moved from its position by 

 pressure of a finger on it, the different points of the land- 

 scape are no longer thrown on identical points of the retinse, 

 and they are therefore seen double. When the eyes converge 

 on an object in front of another, the images of the hinder 

 object are thrown on the inner halves of both retinae, and 

 therefore on points not identical. But, in looking through 

 the stereoscope, although the pictures are two, each eye is 

 directed full on the picture opposite it, and thus correspond- 



