540 THE NERVOUS SYSTEM. 



same refractive power, therefore, brings them together less rapidly 

 than before, and they come to a focus at a greater distance behind the 

 lens. To provide for this, the spy-glass is furnished with a sliding 

 tube, by which the distance of the eye-piece from the object-glass may 

 be shifted at will. For examination of remote objects, the eye-piece is 

 pushed forward, to bring into view the image formed a short distance 

 behind the lens; for that of near objects it is drawn backward, to receive 

 the image placed farther to the rear. This is the accommodation of the 

 spy-glass for vision at different distances. 



There is a similar necessity in the eye. If one eye be covered, and 

 two vertical needles be placed in front of the other, in nearly the same 

 linear range, but at different distances one, for example, at eight, and 

 the other at twenty inches from the eye it will be found that they 



FIG. 139. 



DIAGRAMMATIC SECTION OF THE EYEBALL, showing difference of refraction for direct and indirect 

 vision. cr, x. Rays from a point in the line of direct vision, focussed at the retina, b, y, z. Rays 

 from a point outside the line of direct vision, brought to a focus and dispersed before reaching 

 the retina. 



cannot both be seen distinctly at the same time. When we look at 

 the one nearer the eye, so as to perceive its form distinctly, the image 

 of the more remote one is confused ; and when we see the more dis- 

 tant object in perfection, that which is nearer loses its sharpness of 

 outline. 



The same thing may be shown by stretching in front of the eye, at 

 the distance of seven or eight inches, a gauze veil, or other woven 

 fabric of fine threads, with tolerably open meshes, so that objects may 

 be visible through its tissue. The observer, in using a single eye, may 

 fix at will either the threads of the veil, or the objects beyond it ; but 

 they alternate with each other in distinctness, like the two needles in 

 the foregoing experiment. When the threads are sharply defined, every- 

 thing else is indistinct ; and when the eye is fixed on the more distant 

 objects, so that they are sharply delineated in the field of vision, the 



