128 



ANIMAL MECHANISM. 



FlQ. 14. 



Explanation of Figure 14. 



In this figure, B, B, the eyes, 

 having their axes directed to A, 

 will see the object C, double, 

 somewhere near the outline D, D. 

 Because the line of the direction 

 of the rays from C, do not strike 

 the retina in the same relation to 

 the axis A, B, in both eyes. If a 

 candle is placed at the distance of 

 ten feet, and 1 hold my finger at 

 arm's length, between the eye 

 and the candle, when I look at the 

 candle, my finger appears double, 

 and when I look at the finger, the 

 candle is double. 



Explanation of Figure 15. 



A is exactly in the centre of 

 the axes of both eyes ; consequent- 

 ly it is distinctly seen, and it also 

 appears single,' because the form 

 of it strikes upon the points of the 

 retina, opposite to the pupils in 

 both eyes. Those points, as before 

 remarked, have a correspondence, 

 and the object, instead of appear 

 ing double, is only strengthened in 

 the liveliness of the image. Again, 

 the object B will be seen fainter, 

 but single and correct. It will 

 appear fainter, because there is 

 only one spot in each eye, which 

 possesses the degree of sensibility 

 necessary to perfect vision : thus, 

 it will be understood, the object will appear single, as the rays of 

 light proceeding from it have exactly the same relation to the 

 centre of the retinas, in both eyes. 



THE REASONS WHY CROSS-EYED PERSONS SEE ONLY WITH 

 ONE EYE. 



With such as have a permanent squint, (cross-eye,) 

 only one eye is attended to, though they may not be ap- 

 prehensive of the fact. From continued neglect, the 

 distorted organ wanders farther and farther from the axis 



