SINGLE VISION. 17 



centres of the spheres, to which the cornea, the 

 ball of the eye, and the two portions of the 

 crystalline belong, are all placed in the same 

 right line, hence called the optic axis, and that 

 this being produced both ways, passes through 

 the centres of the cornea and retina, considered 

 as surfaces. Opticians, on their part, observe, 

 that an object appears single to both eyes, when 

 the axis of each is accurately directed to it; 

 from which they infer, that the centres of the 

 retinas agree in suggesting but one object, 

 though each receives its picture. Again ; since 

 it is known by experience, that, while any object 

 is seen single, to which the optic axes are turned, 

 others at the same distance from the eyes like- 

 wise appear so ; and since the pictures of these 

 lateral objects fall upon points in the two retinas, 

 equidistant from their centres, and both upon 

 the same side, that is, both to the right or left 

 of the centres, or both above or below them, 

 opticians conclude, that every two places of the 

 retinas, which are similarly situated with respect 

 to the centres, must also agree in exhibiting 

 but one object, though pictures are received by 

 both. 



But the whole of this reasoning is built upon 

 a circumstance in the fabric of the eye, which 

 has been shown by some of the most eminent 



c 



